On Mon, Nov 14, 2011 at 12:23 PM, Ross Walker <ross.rosswalker.co.uk> wrote:
>> > Will it be alright if I use higher method or basis set to calculate
>> the charges? I saw some documentation that gaff and amber use HF with
>> 6-31g* for the parameters and charges.
>> >
>> > I also check the trajectory, but there seems to be no 'stupid'
>> conformation.
>>
>> The charges are always very sensitive to the ab initio method applied.
>> Try MP2 instead, for example. BTW, 3% higher density is not so much
>> terrible for most tasks.
>
> Be aware that the force field was parameterized for a specific set of
> charges and charge calculation method. If you change this method (e.g.
> changing HF to MP2) then you will likely need to reparameterize the rest of
> the force field to account for this change. It is not a simple matter to
> just change the QM method you use for the charge calculation so I would only
> consider doing this if you really know what you are doing and can carefully
> justify your decision and reasoning when it comes to publication.
If we assume that both HF and MP2 provide an equally good [optimized]
geometries of the system (molecule), than the trick looks painless...
Formally, you are right, of course.
Vitaly
--
Dr. Vitaly V. Chaban, 430 Hutchison Hall, Chem. Dept.
Univ. Rochester, Rochester, New York 14627-0216
THE UNITED STATES OF AMERICA
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Received on Mon Nov 14 2011 - 10:30:04 PST
