Subject: Re: CCL:calculating atomic charges with AIMPAC

From: Jamie Platts <Platts~at~Cardiff.ac.uk>
Date: Thu, 29 May 2003 09:36:18 +0100 (BST)

Dear Julia - have you checked the quality of the integrations? This is
usually best done with the number reported as "L" (I think the fourth
quantity output by proaimv). This is the value of the Laplacian integrated
over the atomic basin, which should vanish - values of 10^-3 are
borderline OK, lower is obviously better. Also check that the total
populations add up to the no. electrons in your system.

  For a system like ferrocene, with rings (and cages?), you might need to
tweak some of the surface-tracing paramaters - see the top of the
proaimv.f code.

  If this isn't the source of your problem, you could try the integration
methods in Gaussian (disappointing in my experience) or Popelier's
MORPHY, which is generally very robust.

  Hope this helps - Jamie

> I currently have an undergraduate student who is attempting to compare
> charges of methylated benzene and ferrocene systems using the AIMPAC
> software.  In the case of hexamethyl benzene and the ferrocene analogues he
> is getting very large negative charges on some the ring carbons (-5 to -20)
> and small positive charges on iron (0.5) Can anyone tell me if there is a
> problem with the program or if the ring system can not be treated with this
> method?
>
> Thank you,
> Julia Metzker
> University of York
> Department of Chemistry

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From: "Qadir K.Timerghazin" <Qadir.Timerghazin:at:CERMM.Concordia.CA>
Date: Thu, 29 May 2003 11:16:39 -0400

Which integration algorithm are you using?
PROAIM is much faster than PROMEGA, but if something goes wrong, I
recommend to compare the results with PROMEGA.
And, of course, always check if your integrated L values are small
enough (at least 10e-3)

-- 
     Qadir                      mailto:Qadir.Timerghazin:at:CERMM.Concordia.CA

SD> But a charge of -20 almost certainly is the result of assigning the
SD> electrons of one atom's basin to another atom.  Something which occurs as a
SD> result of the program failing to find the boundary between them.  

SD> At any rate, I would try a 2nd AIM calculation program if one is available,
SD> before giving up.  E.g., I often find Gaussian's AIM code halts with a
SD> (usually cryptic) error message, but for the same molecule AIM2000 may
SD> complete the calculation with no problem.

SD> -----Original Message-----
SD> From: Wai-To Chan [mailto:chan:at:curl.gkcl.yorku.ca]
SD> Sent: Thursday, May 29, 2003 2:09 AM
SD> To: chemistry:at:ccl.net
SD> Subject: CCL:calculating atomic charges with AIMPAC

SD> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
SD> I currently have an undergraduate student who is attempting to compare 
SD> charges of methylated benzene and ferrocene systems using the AIMPAC 
SD> software.  In the case of hexamethyl benzene and the ferrocene analogues he 
SD> is getting very large negative charges on some the ring carbons (-5 to -20) 
SD> and small positive charges on iron (0.5) Can anyone tell me if there is a 
SD> problem with the program or if the ring system can not be treated with this 
SD> method?

SD>    I remember reading about the tendency of AIM changes to be larger in
SD> magnitude
SD> than expected on the basis of chemical intuition. There is a paper by
SD> Kenneth Wiberg from the early 90s in J. comp. chem. about the comparison of 
SD> several atomic-charge partitioning scheme including Bader's you may find
SD> useful. 
SD> You may also consider using the Natural atomic charges method if you want 
SD> faster and less 'surprising' results. 

SD> Wai-To Chan

SD> To send e-mail to subscribers of CCL put the string CCL: on your Subject:
SD> line

SD> Send your subscription/unsubscription requests to: CHEMISTRY-REQUEST:at:ccl.net

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From: Victor Lua~na <pueyo|at|pinon.ccu.uniovi.es>
Date: Thu, 29 May 2003 19:56:58 +0200 (METDST)

> From: "Dr. Julia Metzker" <jm45|at|york.ac.uk>
> Date: Wed, 28 May 2003 15:13:03 +0100
> Subject: CCL:calculating atomic charges with AIMPAC
>

The charge integrated on the basin of a carbon atom goes from -5 to
-20 electrons??? That looks as big mistake. Either, the wavefunction
is not converged and it is made mostly of noise or the integration has
completely failed in AIMPAC. I suggest you first examining the actual
topology of the electron density. EXTREME is pretty fast and you can
easily discard any wrong-looking wavefunction. If everythhing looks
right you can try the *two* different integration techniques included
within AIMPAC. See the documentation of PROAIM. Notice that the laplacian,
L(r), should integrate to zero within a basin. An integral of 1e-4 a.u.
is reasonable given the finite precision of numerical integrations.
A value larger than some 1e-3 a.u. should make you suspect an error.

      Regards,
                Victor Lua~na

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---

From: Victor Lua~na <pueyo|at|pinon.ccu.uniovi.es>
Date: Thu, 29 May 2003 20:17:18 +0200 (METDST)

> From owner-chemistry|at|ccl.net Thu May 29 08:35:48 2003
> From: Wai-To Chan <chan|at|curl.gkcl.yorku.ca>
> Date: Thu, 29 May 2003 02:09:19 -0400 (EDT)
> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> I remember reading about the tendency of AIM changes to be larger in magnitude
> than expected on the basis of chemical intuition. There is a paper by
> Kenneth Wiberg from the early 90s in J. comp. chem. about the comparison of 
> several atomic-charge partitioning scheme including Bader's you may find useful. 
> You may also consider using the Natural atomic charges method if you want 
> faster and less 'surprising' results. 

What "chemical intuition"? The one that comes from looking for many years
to Mulliken populations in gaussian outputs? That is not science, it is
prejudgment. Whatever interpretation we want to give to them, Bader charges
are unique and well defined properties of the electron density of the
molecule or solid under study. Both experimental and good theoretical
electron densities agree on the calculated Bader charges of hundreths of
compounds well characterized so far. On the other hand, Bader charges
are not enough to analyze the electron densities. The whole multipolar
series (charges are nothing but the zero order poles) should be considered
instead. Several recent works by Popelier, for instance, would made a
good reading in this respect.

In any case, the charges quoted in the original question look like if
the wavefunction is wrong by some reason. Perhaps a calculation
not converged.

   Regards,
             Victor Lua~na