Dear Ben
I would make the strong claim that the absolute energy is pretty
irrelevant. I would only look at relative energies. In that case, while
your argument still holds in principle, it is clear that we will be
talking about meV and not millions...
On 9/2/10 7:33 PM, Ben Roberts wrote:
> Hi all,
>
> The other day a colleague and I were working on a QM/MM calculation using Amber and PUPIL, and we got a large (~40 kcal/mol) discrepancy between, on the one hand, the SCF energy (reported as PUPESCF) in Amber, and on the other, the energy reported by Gaussian and converted from Hartrees into kcal/mol using current NIST conversion factors (see www.nist.gov).
>
> It appears that most of the discrepancy comes from the value of EV_TO_KCAL in Amber, which is currently set to 23.061 kcal mol^-1 eV^-1, a value commented as "Dynamo's conversion". Using the current NIST values, though, the following conversion factors are obtained:
>
> Thermochemical calorie (4.184 J/cal by definition): 23.0605497
> 20-degree calorie (c. 4.182 J/cal): 23.0715781
>
> Other definitions of the calorie I've been able to find (e.g., the 15-degree and international steam table calories) contain more J/cal, and therefore will reduce the kcal mol^-1 eV^-1 if used. Given this observation, I guess that the thermochemical calorie has been used in Amber (also cf. the value of J_PER_CAL).
>
> When the numbers involved are large (in the millions of kcal/mol), the difference of ~0.0005 J/cal becomes important.
>
> EV_TO_KCAL seems to be used only when an energy value is coming down from QM/MM. Perhaps the Amber QM/MM algorithm has been optimised with that value in mind. Is it better, then, to revise the value for everything, or only for those calculations that use PUPIL?
>
> Thanks,
> Ben
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--
Dr. Adrian E. Roitberg
Associate Professor
Quantum Theory Project
Department of Chemistry
Senior Editor. Journal of Physical Chemistry
American Chemical Society
University of Florida PHONE 352 392-6972
P.O. Box 118435 FAX 352 392-8722
Gainesville, FL 32611-8435 Email adrian.qtp.ufl.edu
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Received on Thu Sep 02 2010 - 11:00:06 PDT
