Subject: NEGATIVE ACTIVATION E's

From: "E. Lewars" <[email-masked]>
Date: Mon, 3 Aug 1998 13:32:52 -0400 (EDT)

Monday 1998 August 3

Hello,

Sometimes one gets a _negative_ value for an activation E.  For example

                  O               O
                   \\              \\...
                    C---O           C---O
                    |   |  ----->   .   . transition state   --> CO2 + N2
                    N===N           .   .
                                    .....
                                    N====N
                -297.56194 h
                                     -297.56281 h

      activ E = -0.00087 h = -2.28 kJ/mol     G2(MP2) energies

 Sometimes one gets a negative value with and without the ZPE corection.

 Is there any special theoretical significance to such "impossible"
activation E's?  Or can one just say that (since not much below zero) they
are compatible with a low positive barrier?

     Thanks

              E. Lewars
==============

---

From: Per-Ola Norrby <[email-masked]>
Date: Mon, 3 Aug 1998 22:51:25 +0200

>are compatible with a low positive barrier?

	Well, in this case, it just means you use an improper method...
The G2 methods are extrapolation methods that depend on the geometry being
well represented by a low level method.  In this case, you are using
HF/6-31G* to locate the TS geometry, I guess this is the source of your
problem.  If you use a method where you calculate the energy at the same
level as your geometry, this type of problem should disappear (there are
other possible sources of apparent negative TS energies, like
precomplexing, but that should not apply in your case).  For some good
possible choices for geometry optimization levels, I'd suggest
MP2/6-311G**, or B3LYP/6-31G* (depending on your computational resources
and/or theological leaning).

	The G2 methods were developed to calculate VERY accurate heats of
formation, but in general, I wouldn't recommend them for TS calculations
(just a personal opinion, I'd like to hear if there is a concensus on this).

	Regards,

	Per-Ola Norrby

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Per-Ola Norrby, [email-masked]
Royal Danish School of Pharmacy, Dept. of Med. Chem.
Universitetsparken 2, DK-2100 Copenhagen, Denmark
Tel: +45-35376777-506, +45-35370850, fax +45-35372209

---

From: Anatoli Korkin <[email-masked]>
Date: Mon, 03 Aug 1998 14:06:56 -0700

Negative activition energies you may get in two cases (may be someone
knows more): 1) There is an intermediate complex between reagents and 
a TS. In this case everything is "normal". Some SN2 gas phase 
reactions have "negative" activitations energies. This led first to a
suspecion among experimentalists, who studied these reactions in
solutions,
that theory was wrong, but later this result has been confirmed for
gas phase reactions. 2) For very low barriers you may get negative
activation energies, if you do (higher level) single point calculations
using geometries optimized at another (lower correlation) approach.
If you optimize a TS and reagents at the same level and there is
no an intemediate mimimum on a reaction pathway you should not get
your electronic energy for TS lower than that one for reagents. Of
course, zero point energy correction may change the sign of the barrier,
if it is very small (e.g. 1 kcal/mol or lower). Anyway at such low
barriers a conventional transition state theory is hardly applicable,
and a dynamical treatment is required.

Regards,

Anatoli

P.S. We have a paper on CN2O2 species including the complex below.
An activation energy we got at CCSD(T)/TZP//MBPT(3)/6-31G* was 0.5
kcal/mol before and -0.3 kcal/mol after ZPE correction.

E. Lewars wrote:
> -------This is added Automatically by the Software--------
> -- Original Sender Envelope Address: [email-masked]
> -- Original Sender From: Address: [email-masked]
> [email-masked]: Everybody | [email-masked]: Coordinator
> [email-masked]: HELP CHEMISTRY or HELP SEARCH | Gopher: www.ccl.net 73
> Anon. ftp: www.ccl.net   | [email-masked] -- archive search
>              Web: http://www.ccl.net/chemistry.html

-- 
******************************************************************

Anatoli A. Korkin, Ph.D         Predictive Engineering Lab
Computational Chemist		Semiconductor Products Sector

Motorola Inc., MD M360		Tel:    (602) 655-3171
2200 W. Broadway Road		Fax:    (602) 655-5013
Mesa AZ 85202			E.mail: [email-masked]
					
*******************************************************************