Ross was referring NOT to the quality of MP2 versus the one with HF, but
rather on the use of the electrostatic potential in both methods. to fit
charges using RESP.
This is a potential issue, since the shape and strength of the
electrostatic potential could be very method and basis set dependent. A
priori of course, the MP2 values are 'better', but since the rest of the
force field used hf/6-31G*, you do not want to 'unbalance' it by using
different methods for different parts of the system.
Adrian
On 11/14/11 1:26 PM, Dr. Vitaly V. Chaban wrote:
> 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. Adrian E. Roitberg
Professor
Quantum Theory Project, Department of Chemistry
University of Florida
roitberg.ufl.edu
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Received on Mon Nov 14 2011 - 11:00:03 PST
