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Hi Jee,<br>
<br>
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. <br>
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. (?)<br>
<br>
Cheers, Lars<br>
<br>
<br>
Jee E Rim wrote:<br>
<blockquote cite="mid001301c442a8$9b139fb0$c3a140ab@phoenix" type="cite">
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<div><font face="Arial" size="2">Hi,</font></div>
<div> </div>
<div><font face="Arial" size="2">I'm trying to put together a
topology for a nicotine analogue molecule using the OPLS forcefield.</font></div>
<div> </div>
<div><font face="Arial" size="2">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.</font></div>
<div> </div>
<div><font face="Arial" size="2">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? </font></div>
<div> </div>
<div><font face="Arial" size="2">Thanks for your help in advance!</font></div>
<div><font face="Arial" size="2">Jee.</font></div>
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<pre class="moz-signature" cols="72">--
*******************************************************
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: <a class="moz-txt-link-abbreviated" href="mailto:Lars.Schaefer@mpi-bpc.mpg.de">Lars.Schaefer@mpi-bpc.mpg.de</a>
Home: <a class="moz-txt-link-freetext" href="http://wwwuser.gwdg.de/~lschaef2/homepage.html">http://wwwuser.gwdg.de/~lschaef2/homepage.html</a>
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