[gmx-users] OPLS question

[Jee E Rim]

Lars and David,

Thanks so much for the input! 
And Lars, that means I'll have to subtract the 1-4 interaction energy from the QM total before ls-fitting to the RH dihedral potential form? I'll try doing that.

Thanks,
Jee.

  ----- Original Message ----- 
  From: Lars Schaefer 
  To: Discussion list for GROMACS users 
  Sent: Wednesday, May 26, 2004 12:35 AM
  Subject: Re: [gmx-users] OPLS question


  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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Lars Schaefer
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Theoretische und computergestuetzte Biophysik
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