> > 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.
All the best
Ross
/\
\/
|\oss Walker
---------------------------------------------------------
| Assistant Research Professor |
| San Diego Supercomputer Center |
| Adjunct Assistant Professor |
| Dept. of Chemistry and Biochemistry |
| University of California San Diego |
| NVIDIA Fellow |
|
http://www.rosswalker.co.uk |
http://www.wmd-lab.org/ |
| Tel: +1 858 822 0854 | EMail:- ross.rosswalker.co.uk |
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Received on Mon Nov 14 2011 - 09:30:02 PST
