From owner-chemistry@ccl.net Fri Mar 20 02:49:00 2020 From: "Sebastian Kozuch seb.kozuch_._gmail.com" To: CCL Subject: CCL: Negative activation energies Message-Id: <-54002-200320021407-7690-2UomeKMT/d65wo4fdU7gsA^server.ccl.net> X-Original-From: Sebastian Kozuch Content-Language: en-US Content-Transfer-Encoding: 8bit Content-Type: text/html; charset=utf-8 Date: Fri, 20 Mar 2020 08:13:59 +0200 MIME-Version: 1.0 Sent to CCL by: Sebastian Kozuch [seb.kozuch..gmail.com] 4) Some very low barriers in the energy surface, especially for H transfer type of reactions, may have large ZPE that can be above the TS. This gives a negative Ea when considering Gibbs energies, enthalpies or just E+thermal corrections. While the ZPE might really be in some almost isothermal reactions above the TS, in significantly exothermal reactions it is just an artifact of using an harmonic potential to compute the vibrational frequencies.

5) Before reaching conclusions, you must be certain on what is assumed to be the activation energy. In many computational works it is just DE‡ = E(TS)-E(Int), in which case a negative Ea means a lousy computation, but sometimes H or G is considered (DH‡ and DG‡), and every one will give some different reason for the negative Ea. But you also have the "correct" activation energy that is taken from Arrhenius equation [k=A.exp(-Ea/RT)] as a derivative: Ea =- R. d(ln k)/d(1/T) = DH‡ + RT (this is obtained by derivation of Eyring's TST). There are many confusions on this, and a good paper should define what they mean by Ea to avoid confusions.

Best,
Sebastian

On 20/3/20 1:23 AM, David Shobe shobedavid _ gmail.com wrote:

3) If the TS and reactants are geometry optimized at one level, and then single point energies calculated at a higher level, one can get a negative activation energy. Of course, this is a computational artifact.

--David Shobe

On Thu, Mar 19, 2020, 6:13 PM Eric V. Patterson eric.patterson|,|stonybrook.edu <owner-chemistry * ccl.net> wrote:

Hello Sam,

There are at least a couple of ways that a negative activation barrier might be reported from a computational method. The two I see most often both involve errors in treating the system or interpreting the data.

1) The barrier has been measured from reactants at infinite separation rather > from a pre-reaction complex that is lower in energy than the separated reactants. I’ve seen this happen most often when people have failed to confirm their TS via IRC and incorrectly assume the reference point.

2) The barrier is computed from free energies, and there is some significant error in the free energy corrections. This could be from large errors in entropy, not properly accounting for standard states in bimolecular reactions, or so forth.

I’m sure there are other cases.

I hope you’re well,

Eric

----------------------------------------------------
Eric V. Patterson, PhD

Director of Undergraduate Laboratories
Senior Lecturer

On Mar 19, 2020, at 4:02 PM, Abrash, Samuel sabrash,+,richmond.edu <owner-chemistry%a%ccl.net> wrote:

Sent to CCL by: "Abrash, Samuel" [sabrash-.-richmond.edu]
Hi Folks,

I’ve been reading some papers on Criegee intermediates, that talk about an initio and DFT calculations resulting in negative activation energies. I know that experimentally negative activation energies are an artifact observed when the activation barrier is close to zero and the preexponential factor decreases with increasing temperature, for example in cases where the activated complex includes a van der Waals complex.

But I don’t understand how a theoretical calculation of a PES can result in a negative activation energy. Any downhill PES simply has an EA of zero. The only answer I can think of is if the authors are taking their PES and calculating the EA using Eyring theory or the equivalent and that the Eyring calculation has a preexpontial factor that decreases with increasing T. (They don’t deign to explain how the negative EAs are obtained).

If anyone can enlighten me it would be much appreciated.

Best regards,
Sam Abrash
Associate Professor
University of Richmond

the strange characters on the top line to the %a% sign. You can also

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From owner-chemistry@ccl.net Fri Mar 20 03:32:01 2020 From: "Ramon Crehuet ramon.crehuet * iqac.csic.es" To: CCL Subject: CCL: Negative activation energies Message-Id: <-54003-200320032851-25432-C8g0CzEvj1la+nnHCuT/uQ*o*server.ccl.net> X-Original-From: Ramon Crehuet Content-Type: multipart/alternative; boundary="----=_Part_1553181_474814532.1584689318283" Date: Fri, 20 Mar 2020 08:28:38 +0100 (CET) MIME-Version: 1.0 Sent to CCL by: Ramon Crehuet [ramon.crehuet%x%iqac.csic.es] ------=_Part_1553181_474814532.1584689318283 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Hi Sam,=20 I agree with Eric. Concerning his 1st point, consider also the effect of ba= sis set superposition error, which artificially stabilizes transition state= s with respect to more separated (or infinitely separated) species.=20 And there is one more case that I can think of:=20 3) Often energies are reported as single point calculations with a higher q= uality methods on geometries optimized on a lower quality method. This mean= s that the reported energies do not correspond exactly to the stationary po= ints of the higher quality method.=20 Hope this helps.=20 Ramon=20 ----- On 19 Mar, 2020, at 22:47, Eric V. Patterson eric.patterson|,|stonybr= ook.edu wrote:=20 > Hello Sam, > There are at least a couple of ways that a negative activation barrier mi= ght be > reported from a computational method. The two I see most often both invol= ve > errors in treating the system or interpreting the data. > 1) The barrier has been measured from reactants at infinite separation ra= ther > > from a pre-reaction complex that is lower in energy than the separated > reactants. I=E2=80=99ve seen this happen most often when people have fail= ed to confirm > their TS via IRC and incorrectly assume the reference point. > 2) The barrier is computed from free energies, and there is some signific= ant > error in the free energy corrections. This could be from large errors in > entropy, not properly accounting for standard states in bimolecular react= ions, > or so forth. > I=E2=80=99m sure there are other cases. > I hope you=E2=80=99re well, > Eric > ---------------------------------------------------- > Eric V. Patterson, PhD > Director of Undergraduate Laboratories > Senior Lecturer >> On Mar 19, 2020, at 4:02 PM, Abrash, Samuel sabrash,+, richmond.edu < >> owner-chemistry%a%ccl.net > wrote: >> Sent to CCL by: "Abrash, Samuel" [sabrash-.- richmond.edu ] >> Hi Folks, >> I=E2=80=99ve been reading some papers on Criegee intermediates, that tal= k about an >> initio and DFT calculations resulting in negative activation energies. I= know >> that experimentally negative activation energies are an artifact observe= d when >> the activation barrier is close to zero and the preexponential factor de= creases >> with increasing temperature, for example in cases where the activated co= mplex >> includes a van der Waals complex. >> But I don=E2=80=99t understand how a theoretical calculation of a PES ca= n result in a >> negative activation energy. Any downhill PES simply has an EA of zero. T= he only >> answer I can think of is if the authors are taking their PES and calcula= ting >> the EA using Eyring theory or the equivalent and that the Eyring calcula= tion >> has a preexpontial factor that decreases with increasing T. (They don=E2= =80=99t deign >> to explain how the negative EAs are obtained). >> If anyone can enlighten me it would be much appreciated. >> Best regards, >> Sam Abrash >> Associate Professor >> University of Richmond >> -=3D This is automatically added to each message by the mailing script = =3D->> the strange characters on the top line to the %a% sign. You can also >> E-mail to subscribers: CHEMISTRY%a%ccl.net or use:>> E-mail to administrators: CHEMISTRY-REQUEST%a%ccl.net or use------=_Part_1553181_474814532.1584689318283 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: quoted-printable

Hi Sam,

I agree with Eric. Conc= erning his 1st point, consider also the effect of basis set superposition e= rror, which artificially stabilizes transition states with respect to more = separated (or infinitely separated) species.

And there is one more case that I can think of:

3) Often energies are reported as single point calculations wi= th a higher quality methods on geometries optimized on a lower quality meth= od. This means that the reported energies do not correspond exactly to the = stationary points of the higher quality method.

Hope this helps.

Ramon

----- On 19 Mar, 2020, at 22:47, Eric V. Patterson eric.patterson|,= |stonybrook.edu <owner-chemistry^ccl.net> wrote:

Hello Sam,

T= here are at least a couple of ways that a negative activation barrier might= be reported from a computational method. The two I see most often bo= th involve errors in treating the system or interpreting the data.

1) The barrier has been m= easured from reactants at infinite separation rather > from a pre-reacti= on complex that is lower in energy than the separated reactants. I=E2= =80=99ve seen this happen most often when people have failed to confirm the= ir TS via IRC and incorrectly assume the reference point.

2) The barrier is computed from = free energies, and there is some significant error in the free energy corre= ctions. This could be from large errors in entropy, not properly acco= unting for standard states in bimolecular reactions, or so forth.

I=E2=80=99m sure there are= other cases.

I h= ope you=E2=80=99re well,

Eric

-------= ---------------------------------------------
Eric V. Patters= on, PhD

Director o= f Undergraduate Laboratories
Senior Lecturer

On Mar 19, 2020,= at 4:02 PM, Abrash, Samuel sabrash,+,richmond.edu <owner-chemistry%a%ccl.net&g= t; wrote:

Sent to CCL by: "Abrash, Samuel" [sabrash-.-richmond.edu]<= br class=3D"">Hi Folks,

I=E2=80=99ve been read= ing some papers on Criegee intermediates, that talk about an initio and DFT= calculations resulting in negative activation energies. I know that = experimentally negative activation energies are an artifact observed when t= he activation barrier is close to zero and the preexponential factor decrea= ses with increasing temperature, for example in cases where the activated c= omplex includes a van der Waals complex.

But I= don=E2=80=99t understand how a theoretical calculation of a PES can result= in a negative activation energy. Any downhill PES simply has an EA o= f zero. The only answer I can think of is if the authors are taking t= heir PES and calculating the EA using Eyring theory or the equivalent and t= hat the Eyring calculation has a preexpontial factor that decreases with in= creasing T. (They don=E2=80=99t deign to explain how the negative EAs are o= btained).

If anyone can enlighten me it would = be much appreciated.

Best regards,
Sam Abrash
Associate Professor
University o= f Richmond

-=3D = This is automatically added to each message by the mailing script =3D-
To recover the email address of the author of the message, please= change
the strange characters on the top line to the %a% sig= n. You can also

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------=_Part_1553181_474814532.1584689318283-- From owner-chemistry@ccl.net Fri Mar 20 06:27:00 2020 From: "Mark Zottola mzottola.:.gmail.com" To: CCL Subject: CCL: Negative activation energies Message-Id: <-54004-200320062407-13681-SoPr1Opz9ie536QCa4oIkw##server.ccl.net> X-Original-From: Mark Zottola Content-Type: multipart/alternative; boundary="000000000000f743fd05a146ae06" Date: Fri, 20 Mar 2020 19:23:47 +0900 MIME-Version: 1.0 Sent to CCL by: Mark Zottola [mzottola ~ gmail.com] --000000000000f743fd05a146ae06 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Ramon, I would want to amplify your point that calculation of a structure at a lower basis set then a single point at a higher basis set is foolishness. Each basis set has their own definition of the potential energy surface. Sometimes the differences in structure (therefore energy) are small, yet often those differences are large. If you want the energies from a higher level of basis set then bite the bullet and be consistent with the calculations. My colleague and I touched on this in Chemical Physical Letters, pgs 254-285, Volume 419 issues 1-3, 2006. On Fri, Mar 20, 2020 at 6:57 PM Ramon Crehuet ramon.crehuet * iqac.csic.es = < owner-chemistry[#]ccl.net> wrote: > Hi Sam, > I agree with Eric. Concerning his 1st point, consider also the effect of > basis set superposition error, which artificially stabilizes transition > states with respect to more separated (or infinitely separated) species. > And there is one more case that I can think of: > 3) Often energies are reported as single point calculations with a higher > quality methods on geometries optimized on a lower quality method. This > means that the reported energies do not correspond exactly to the > stationary points of the higher quality method. > Hope this helps. > Ramon > > ----- On 19 Mar, 2020, at 22:47, Eric V. Patterson eric.patterson|,| > stonybrook.edu wrote: > > > Hello Sam, > > > There are at least a couple of ways that a negative activation barrier > might be > > reported from a computational method. The two I see most often both > involve > > errors in treating the system or interpreting the data. > > > 1) The barrier has been measured from reactants at infinite separation > rather > > > from a pre-reaction complex that is lower in energy than the separated > > reactants. I=E2=80=99ve seen this happen most often when people have fa= iled to > confirm > > their TS via IRC and incorrectly assume the reference point. > > > 2) The barrier is computed from free energies, and there is some > significant > > error in the free energy corrections. This could be from large errors i= n > > entropy, not properly accounting for standard states in bimolecular > reactions, > > or so forth. > > > I=E2=80=99m sure there are other cases. > > > I hope you=E2=80=99re well, > > Eric > > > ---------------------------------------------------- > > Eric V. Patterson, PhD > > Director of Undergraduate Laboratories > > Senior Lecturer > > >> On Mar 19, 2020, at 4:02 PM, Abrash, Samuel sabrash,+, richmond.edu < > >> owner-chemistry%a%ccl.net > wrote: > > >> Sent to CCL by: "Abrash, Samuel" [sabrash-.- richmond.edu ] > >> Hi Folks, > > >> I=E2=80=99ve been reading some papers on Criegee intermediates, that t= alk about > an > >> initio and DFT calculations resulting in negative activation energies. > I know > >> that experimentally negative activation energies are an artifact > observed when > >> the activation barrier is close to zero and the preexponential factor > decreases > >> with increasing temperature, for example in cases where the activated > complex > >> includes a van der Waals complex. > > >> But I don=E2=80=99t understand how a theoretical calculation of a PES = can > result in a > >> negative activation energy. Any downhill PES simply has an EA of zero. > The only > >> answer I can think of is if the authors are taking their PES and > calculating > >> the EA using Eyring theory or the equivalent and that the Eyring > calculation > >> has a preexpontial factor that decreases with increasing T. (They don= =E2=80=99t > deign > >> to explain how the negative EAs are obtained). > > >> If anyone can enlighten me it would be much appreciated. > > >> Best regards, > >> Sam Abrash > >> Associate Professor > >> University of Richmond > > >> -=3D This is automatically added to each message by the mailing script > =3D->> the strange characters on the top line to the %a% sign. You can al= so > > >> E-mail to subscribers: CHEMISTRY%a%ccl.net or use:>> E-mail to > administrators: CHEMISTRY-REQUEST%a%ccl.net or > use------_Part_1553181_474814532.1584689318283 > Content-Type: text/html; charsetutf-8 > Content-Transfer-Encoding: quoted-printable > >

font-size: 12pt; color: #000000">

Hi Sam,

I agree with > Eric. Concerning his 1st point, consider also the effect of basis set > superposition error, which artificially stabilizes transition states with > respect to more separated (or infinitely separated) species.
data-mce-bogus=3D"1">

And there is one more case that I can thi= nk > of:

3) Often energies are reported as > single point calculations with a higher quality methods on geometries > optimized on a lower quality method. This means that the reported energie= s > do not correspond exactly to the stationary points of the higher quality > method.

Hope this helps.
data-mce-bogus=3D"1">

Ramon
data-mce-bogus=3D"1">

data-marker=3D"__DIVIDER__">----- On 19 Mar, 2020, at 22:47, Eric V. > Patterson eric.patterson|,|stonybrook.edu <owner-chemistry..ccl.net>= ; > wrote:

style=3D"border-left:2px solid > #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;fo= nt-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif= ;font-size:12pt;">Hello > Sam,

There are at lea= st a > couple of ways that a negative activation barrier might be reported from = a > computational method. The two I see most often both involve errors = in > treating the system or interpreting the data.

class=3D"">
1) The barrier has been measured from > reactants at infinite separation rather > from a pre-reaction complex > that is lower in energy than the separated reactants. I=E2=80=99ve = seen this > happen most often when people have failed to confirm their TS via IRC and > incorrectly assume the reference point.

class=3D"">
2) The barrier is computed from free ene= rgies, > and there is some significant error in the free energy corrections. > This could be from large errors in entropy, not properly accounting > for standard states in bimolecular reactions, or so forth.
class=3D"">
I=E2=80=99m sure there ar= e other > cases.

I hope y= ou=E2=80=99re > well,
Eric

>
letter-spacing: normal; text-align: start; text-indent: 0px; > text-transform: none; white-space: normal; word-spacing: 0px; > -webkit-text-stroke-width: 0px; text-decoration: none; word-wrap: > break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" > class=3D"">
word-wrap: break-word; -webkit-nbsp-mode: space; line-break: > after-white-space;" class=3D"">
0px; word-wrap: break-word; -webkit-nbsp-mode: space; line-break: > after-white-space;" class=3D"">
0px; word-wrap: break-word; -webkit-nbsp-mode: space; line-break: > after-white-space;" class=3D"">
0px; word-wrap: break-word; -webkit-nbsp-mode: space; line-break: > after-white-space;" class=3D"">
0px; word-wrap: break-word; -webkit-nbsp-mode: space; line-break: > after-white-space;" class=3D"">
rgb(0, 0, 0); letter-spacing: normal; text-transform: none; white-space: > normal; word-spacing: 0px; text-decoration: none; > -webkit-text-stroke-width: 0px; text-align: start; text-indent: 0px; > word-wrap: break-word; -webkit-nbsp-mode: space; line-break: > after-white-space;" class=3D""> style=3D"font-size: xx-small;" > size=3D"1">----------------------------------------------------
class=3D"">Eric V. Patterson, PhD
0, 0); color: rgb(0, 0, 0); letter-spacing: normal; text-transform: none; > white-space: normal; word-spacing: 0px; text-decoration: none; > -webkit-text-stroke-width: 0px; text-align: start; text-indent: 0px; > word-wrap: break-word; -webkit-nbsp-mode: space; line-break: > after-white-space;" class=3D""> style=3D"font-size: xx-small;" size=3D"1">Director of Undergraduate > Laboratories
Senior Lecturer
style=3D"text-align: start; text-indent: 0px; word-wrap: break-word; > -webkit-nbsp-mode: space; line-break: after-white-space;" class=3D"">
class=3D"">

class=3D"">
>
>

On Mar 19, 202= 0, at > 4:02 PM, Abrash, Samuel sabrash,+, target=3D"_blank">richmond.edu < owner-chemistry%a%ccl.net" class=3D"" target=3D"_blank">owner-chemistry%a= % > ccl.net> wrote:

class=3D"">

Sent to CCL by: "Abrash, Samuel= " > [sabrash-.- > richmond.edu]
Hi Folks,

I= =E2=80=99ve > been reading some papers on Criegee intermediates, that talk about an > initio and DFT calculations resulting in negative activation energies. > I know that experimentally negative activation energies are an > artifact observed when the activation barrier is close to zero and the > preexponential factor decreases with increasing temperature, for example = in > cases where the activated complex includes a van der Waals complex.
class=3D"">
But I don=E2=80=99t understand how a theoretica= l calculation > of a PES can result in a negative activation energy. Any downhill P= ES > simply has an EA of zero. The only answer I can think of is if the > authors are taking their PES and calculating the EA using Eyring theory o= r > the equivalent and that the Eyring calculation has a preexpontial factor > that decreases with increasing T. (They don=E2=80=99t deign to explain ho= w the > negative EAs are obtained).

If anyone can > enlighten me it would be much appreciated.

B= est > regards,
Sam Abrash
Associate Professor
class=3D"">University of Richmond

class=3D"">-=3D This is automatically added to each message by the mailin= g > script =3D-
To recover the email address of the author of t= he > message, please change
the strange characters on the top li= ne > to the %a% sign. You can also

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>

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--000000000000f743fd05a146ae06 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable

Ram= on, I would want to amplify your point that calculation of a structure at a= lower basis set then a single point at a higher basis set is foolishness.= =C2=A0 Each basis set has their own definition of the potential=C2=A0energy= surface.=C2=A0 Sometimes the differences=C2=A0in structure (therefore ener= gy) are small, yet often those differences are large.=C2=A0 If you want the= energies from a higher level of basis set then bite the bullet and be cons= istent with the calculations.

My colleague and I touched on this in Chemical Physical Letters, pgs 2= 54-285, Volume 419 issues 1-3, 2006.=C2=A0=C2=A0

On Fri, Mar 20, 2020 at 6:57 PM Ramon Cre= huet ramon.crehuet * iqac.csic.es <<= a href=3D"mailto:owner-chemistry[#]ccl.net">owner-chemistry[#]ccl.net> w= rote:

Hi Sam, I agree with Eric. Concerning his 1st point, consider also the effect of ba= sis set superposition error, which artificially stabilizes transition state= s with respect to more separated (or infinitely separated) species.
And there is one more case that I can think of:
3) Often energies are reported as single point calculations with a higher q= uality methods on geometries optimized on a lower quality method. This mean= s that the reported energies do not correspond exactly to the stationary po= ints of the higher quality method.
Hope this helps.
Ramon

----- On 19 Mar, 2020, at 22:47, Eric V. Patterson eric.patterson|,|stonybrook.= edu <owner-chemistry..ccl.net> wrote:

> Hello Sam,

> There are at least a couple of ways that a negative activation barrier= might be
> reported from a computational method. The two I see most often both in= volve
> errors in treating the system or interpreting the data.

> 1) The barrier has been measured from reactants at infinite separation= rather >
> from a pre-reaction complex that is lower in energy than the separated=
> reactants. I=E2=80=99ve seen this happen most often when people have f= ailed to confirm
> their TS via IRC and incorrectly assume the reference point.

> 2) The barrier is computed from free energies, and there is some signi= ficant
> error in the free energy corrections. This could be from large errors = in
> entropy, not properly accounting for standard states in bimolecular re= actions,
> or so forth.

> I=E2=80=99m sure there are other cases.

> I hope you=E2=80=99re well,
> Eric

> ----------------------------------------------------
> Eric V. Patterson, PhD
> Director of Undergraduate Laboratories
> Senior Lecturer

>> On Mar 19, 2020, at 4:02 PM, Abrash, Samuel sabrash,+, richmond.edu = <
>> owner-chemistry%a%ccl.net > wrote:

>> Sent to CCL by: "Abrash, Samuel" [sabrash-.- richmond.edu = ]
>> Hi Folks,

>> I=E2=80=99ve been reading some papers on Criegee intermediates, th= at talk about an
>> initio and DFT calculations resulting in negative activation energ= ies. I know
>> that experimentally negative activation energies are an artifact o= bserved when
>> the activation barrier is close to zero and the preexponential fac= tor decreases
>> with increasing temperature, for example in cases where the activa= ted complex
>> includes a van der Waals complex.

>> But I don=E2=80=99t understand how a theoretical calculation of a = PES can result in a
>> negative activation energy. Any downhill PES simply has an EA of z= ero. The only
>> answer I can think of is if the authors are taking their PES and c= alculating
>> the EA using Eyring theory or the equivalent and that the Eyring c= alculation
>> has a preexpontial factor that decreases with increasing T. (They = don=E2=80=99t deign
>> to explain how the negative EAs are obtained).

>> If anyone can enlighten me it would be much appreciated.

>> Best regards,
>> Sam Abrash
>> Associate Professor
>> University of Richmond

>> -=3D This is automatically added to each message by the mailing sc= ript =3D->> the strange characters on the top line to the %a% sign. Y= ou can also

>> E-mail to subscribers: CHEMISTRY%a%ccl.net or use:>> E-mail to adm= inistrators: CHEMISTRY-REQUEST%a%ccl.net or use------_Part_1553181_474814532.15846= 89318283
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<html><body><div style=3D"font-family: arial,helvetica,= sans-serif; font-size: 12pt; color: #000000"><div>Hi Sam,<= br></div><div>I agree with Eric. Concerning his 1st point, c= onsider also the effect of basis set superposition error, which artificiall= y stabilizes transition states with respect to more separated (or infinitel= y separated) species.<br data-mce-bogus=3D"1"></div>&= lt;div>And there is one more case that I can think of:<br data-mce-bo= gus=3D"1"></div><div>3) Often energies are reporte= d as single point calculations with a higher quality methods on geometries = optimized on a lower quality method. This means that the reported energies = do not correspond exactly to the stationary points of the higher quality me= thod.<br data-mce-bogus=3D"1"></div><div>Hope t= his helps.<br data-mce-bogus=3D"1"></div><div>R= amon<br data-mce-bogus=3D"1"></div><div><br&= gt;</div><span id=3D"zwchr" data-marker=3D"__DIVIDE= R__">----- On 19 Mar, 2020, at 22:47, Eric V. Patterson eric.patter= son|,|stonybrook.edu <owner-chemistry..ccl.net> wrote:<br><= /span><div data-marker=3D"__QUOTED_TEXT__"><blockquot= e style=3D"border-left:2px solid #1010FF;margin-left:5px;padding-left:= 5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;fo= nt-family:Helvetica,Arial,sans-serif;font-size:12pt;">Hello Sam,<= ;div class=3D""><br class=3D""></div><= ;div class=3D"">There are at least a couple of ways that a neg= ative activation barrier might be reported from a computational method. &am= p;nbsp;The two I see most often both involve errors in treating the system = or interpreting the data.</div><div class=3D""><br= class=3D""></div><div class=3D"">1) The= barrier has been measured from reactants at infinite separation rather &am= p;gt; from a pre-reaction complex that is lower in energy than the separate= d reactants.  I=E2=80=99ve seen this happen most often when people= have failed to confirm their TS via IRC and incorrectly assume the referen= ce point.</div><div class=3D""><br class=3D"&= quot;></div><div class=3D"">2) The barrier is comp= uted from free energies, and there is some significant error in the free en= ergy corrections.  This could be from large errors in entropy, not= properly accounting for standard states in bimolecular reactions, or so fo= rth.</div><div class=3D""><br class=3D""= ></div><div class=3D"">I=E2=80=99m sure there are = other cases.</div><div class=3D""><br class=3D&quo= t;"></div><div class=3D"">I hope you=E2=80=99= re well,</div><div class=3D"">Eric</div><div= class=3D""><br class=3D""><div class=3D&qu= ot;">
<div dir=3D"auto" style=3D"caret-color: rgb(0, 0, 0); col= or: rgb(0, 0, 0); letter-spacing: normal; text-align: start; text-indent: 0= px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-t= ext-stroke-width: 0px; text-decoration: none; word-wrap: break-word; -webki= t-nbsp-mode: space; line-break: after-white-space;" class=3D"&quo= t;><div dir=3D"auto" style=3D"text-align: start; text-= indent: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; line-break: a= fter-white-space;" class=3D""><div style=3D"text-= align: start; text-indent: 0px; word-wrap: break-word; -webkit-nbsp-mode: s= pace; line-break: after-white-space;" class=3D""><div = style=3D"text-align: start; text-indent: 0px; word-wrap: break-word; -= webkit-nbsp-mode: space; line-break: after-white-space;" class=3D"= ;"><div style=3D"text-align: start; text-indent: 0px; word-= wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;&= quot; class=3D""><div style=3D"text-align: start; text= -indent: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; line-break: = after-white-space;" class=3D""><div style=3D"care= t-color: rgb(0, 0, 0); color: rgb(0, 0, 0); letter-spacing: normal; text-tr= ansform: none; white-space: normal; word-spacing: 0px; text-decoration: non= e; -webkit-text-stroke-width: 0px; text-align: start; text-indent: 0px; wor= d-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space= ;" class=3D""><span data-mce-style=3D"font-size: = xx-small;" style=3D"font-size: xx-small;" size=3D"1&quo= t;>----------------------------------------------------<br class=3D&q= uot;">Eric V. Patterson, PhD</span></div><div style= =3D"caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0); letter-spacing: no= rmal; text-transform: none; white-space: normal; word-spacing: 0px; text-de= coration: none; -webkit-text-stroke-width: 0px; text-align: start; text-ind= ent: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; line-break: afte= r-white-space;" class=3D""><span data-mce-style=3D&quo= t;font-size: xx-small;" style=3D"font-size: xx-small;" size= =3D"1">Director of Undergraduate Laboratories<br class=3D&q= uot;">Senior Lecturer</span></div><div style=3D"= ;text-align: start; text-indent: 0px; word-wrap: break-word; -webkit-nbsp-m= ode: space; line-break: after-white-space;" class=3D"">&l= t;br class=3D""><br class=3D""><br class=3D= ""><br class=3D""></div></div><= /div></div></div></div></div>
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<div><br class=3D""><blockquote class=3D"&quo= t;><div class=3D"">On Mar 19, 2020, at 4:02 PM, Abrash, = Samuel sabrash,+,<a href=3D"http://richmond.edu" class=3D"&= quot; target=3D"_blank">richmond.edu</a> <<a hre= f=3D"mailto:owner-chemistry%a%ccl.net" class=3D"" target=3D= "_blank">owner-chemistry%a%ccl.net</a>> wrote:</div&g= t;<br class=3D"Apple-interchange-newline"><div class=3D&= quot;"><div class=3D""><br class=3D""&= gt;Sent to CCL by: "Abrash, Samuel" [sabrash-.-<a href=3D"= ;http:= //richmond.edu" class=3D"" target=3D"_blank"&g= t;rich= mond.edu</a>]<br class=3D"">Hi Folks,<br class= =3D""><br class=3D"">I=E2=80=99ve been reading= some papers on Criegee intermediates, that talk about an initio and DFT ca= lculations resulting in negative activation energies.  I know that= experimentally negative activation energies are an artifact observed when = the activation barrier is close to zero and the preexponential factor decre= ases with increasing temperature, for example in cases where the activated = complex includes a van der Waals complex.<br class=3D""><= ;br class=3D"">But I don=E2=80=99t understand how a theoretica= l calculation of a PES can result in a negative activation energy. &nbs= p;Any downhill PES simply has an EA of zero.  The only answer I ca= n think of is if the authors are taking their PES and calculating the EA us= ing Eyring theory or the equivalent and that the Eyring calculation has a p= reexpontial factor that decreases with increasing T. (They don=E2=80=99t de= ign to explain how the negative EAs are obtained).<br class=3D"&quo= t;><br class=3D"">If anyone can enlighten me it would be= much appreciated.<br class=3D""><br class=3D""= ;>Best regards,<br class=3D"">Sam Abrash<br class=3D&= quot;">Associate Professor <br class=3D"">Universit= y of Richmond<br class=3D""><br class=3D"">= <br class=3D""><br class=3D"">-=3D This is = automatically added to each message by the mailing script =3D-<br class= =3D"">To recover the email address of the author of the messag= e, please change<br class=3D"">the strange characters on th= e top line to the %a% sign. You can also<br class=3D""><= br class=3D"">E-mail to subscribers: <a href=3D"mailto= :CHEMISTRY%a%c= cl.net" class=3D"" target=3D"_blank">CHEMIS= TRY%a%ccl.n= et</a> or use:<br class=3D"">  &nbsp= ;   <a href=3D"htt= p://www.ccl.net/cgi-bin/ccl/send_ccl_message" class=3D""= target=3D"_blank">http://www.ccl.net/cg= i-bin/ccl/send_ccl_message</a><br class=3D""><= br class=3D"">E-mail to administrators: <a href=3D"mai= lto:CH= EMISTRY-REQUEST%a%ccl.net" class=3D"" target=3D"_bl= ank">CHEMISTRY-REQUEST%a%ccl.net</a> or use<br class=3D"&quo= t;>      <a href=3D"= ;http://www.ccl.net/cgi-bin/ccl/send_ccl_message&= quot; class=3D"" target=3D"_blank">http://www.ccl.net/cgi-bin/ccl/send_ccl_message</a><br cl= ass=3D""><br class=3D"">   &= amp;nbsp;  <a href=3D"http://w= ww.ccl.net/chemistry/sub_unsub.shtml" class=3D"" target= =3D"_blank">http://www.ccl.net/chemistry/su= b_unsub.shtml</a><br class=3D""><br class=3D&q= uot;">Before posting, check wait time at: <a href=3D"http://www.cc= l.net" class=3D"" target=3D"_blank">http://www.ccl= .net</a><br class=3D""><br class=3D""= ;>Job: <a href=3D"http://www.ccl.net/jobs" class=3D"= " target=3D"_blank">http://www.ccl.net/jobs</a> &= lt;br class=3D"">Conferences: <a href=3D"http://server.ccl.net/chemistry/announcements/conferenc= es/" class=3D"" target=3D"_blank">http://server.ccl.net/chemistry/announcements/con= ferences/</a><br class=3D""><br class=3D"= ">Search Messages: <a href=3D"http= ://www.ccl.net/chemistry/searchccl/index.shtml" class=3D"&quo= t; target=3D"_blank">http://www.ccl.n= et/chemistry/searchccl/index.shtml</a><br class=3D""= ><br class=3D""<br class=3D"">  &a= mp;nbsp;   <a href=3D"http://www.c= cl.net/spammers.txt" class=3D"" target=3D"_blank&qu= ot;>http://www.ccl.net/spammers.txt</a><br class=3D&q= uot;"><br class=3D"">RTFI: <a href=3D"http://www.ccl.net/chemistry/aboutccl/instructions/" class=3D"" target=3D"_blank">http://www.ccl.net/chemistry/aboutccl/instructions/</a&g= t;<br class=3D""><br class=3D""><br clas= s=3D""></div></div></blockquote></div>= <br class=3D""></div><br></blockquote><= ;/div></div></body></html>

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