[gmx-users] OPLS question

[Lars Schaefer]

Hi Jee,

as DvdS stated in this list some days ago, you DO have to include the 
1-4 [ pairs ] for the opls FF additionally, although you use RBDIHS. The 
manual has to be updated in this respect.
To my opinion, this also means that you have to think about your 
ls-fitting-procedure, because also the 1-4 interactions contribute to 
the overall roatational barrier about the specific bonds you 
investigate, not only the RB potential (you have to hit the QM 
electronic energy with the total  FF energy, which - also for the 
rotation about the bonds - constitutes more than just the RB potential). 
However, it might well be a good 1st approach, I guess. (?)

Cheers, Lars


Jee E Rim wrote:

> Hi,
>  
> I'm trying to put together a topology for a nicotine analogue molecule 
> using the OPLS forcefield.
>  
> Most of the atom types that I need and the force field parameters are 
> already in the OPLS force field, except for a couple of dihedrals. So, 
> I ran a series of single point calculations using Gaussian98 to get 
> the energies of the configuration with the dihedral angles changing 
> from 0 to 360, and am attempting to get the dihedral coefficients by a 
> least-squares fitting to the Ryckaert-Bellemans potential function. Is 
> this a good way to get the dihedral parameters? I'm using the 
> optimized geometry I get from Gaussian, and am just changing the 
> dihedral angles.
>  
> The second question is, in the manual it's stated that, with the RB 
> potential, the 1-4 interactions must be excluded, is this also the 
> case when you're using the OPLS force field? And should I also exclude 
> the 1-4 interactions when I'm using the parameters derived as above?
>  
> Thanks for your help in advance!
> Jee.
>
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-- 
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Lars Schaefer
MPI fuer biophysikalische Chemie
Theoretische und computergestuetzte Biophysik
Abteilung 070
Am Fassberg 11, D-37077 Goettingen
Tel: 0049 551 201-2305, Fax: 0049 551 201-2302
E-Mail: Lars.Schaefer at mpi-bpc.mpg.de
Home: http://wwwuser.gwdg.de/~lschaef2/homepage.html
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