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</style><pre>Hi</pre><pre> </pre><pre>I've converted the OPLS-AA torsional potential parameters for the</pre><pre>alkane C-C-C-C (CT-CT-CT-CT in the gromacs parameter file notation),</pre><pre>C-C-C-H (CT-CT-CT-HC), and H-C-C-H (HC-CT-CT-HC) torsions from the</pre>
<pre>OPLS format given in Jorgensen et al, JACS 118, 11225 (1996) </pre><pre>to the Ryckaert-Bellemans format given in ffoplsaabon.itp and found</pre><pre>that the calculated values are different.</pre><pre> </pre><pre>A previous post to the gmx-users mailing list on March 27, 2008,</pre>
<pre>pointed out this issue for the H-C-C-H torsional potential but there was no response to that.</pre><pre> </pre><pre>Does anyone know if there is an error in the ffoplsaabon.itp file? Or</pre><pre>is there a newer set of OPLS-AA parameters?</pre>
<pre> </pre><pre>For the OPLS-AA parameters (in kcal/mol), I used:</pre><pre> </pre><pre>dihedral<span style=""> </span>V1<span style=""> </span>V2<span style=""> </span>V3</pre><pre>C-C-C-C<span style=""> </span>1.740<span style=""> </span>-0.157<span style=""> </span>0.279</pre>
<pre>C-C-C-H<span style=""> </span>0.0<span style=""> </span>0.0<span style=""> </span>0.366</pre><pre>H-C-C-H<span style=""> </span>0.0<span style=""> </span>0.0<span style=""> </span>0.318</pre>
<pre> </pre><pre>from which I calculated the Ryckaert-Bellemans parameters (in kJ/mol) as:</pre><pre> </pre><pre>dihedral<span style=""> </span>C0<span style=""> </span><span style=""> </span>C1<span style=""> </span>C2<span style=""> </span>C3</pre>
<pre>C-C-C-C<span style=""> </span>3.56686<span style=""> </span>-1.889076<span style=""> </span>0.65688 <span style=""> </span>-2.33467</pre><pre>C-C-C-H<span style=""> </span>0.66526<span style=""> </span>1.99577<span style=""> </span>0.0<span style=""> </span>-2.661024</pre>
<pre>H-C-C-H<span style=""> </span>0.76567<span style=""> </span>-2.29702<span style=""> </span>0.0<span style=""> </span>-3.06269</pre><pre> </pre><pre>the parameters in the ffoplsaabon.itp file are:</pre>
<pre> </pre><pre>dihedral<span style=""> </span>C0<span style=""> </span>C1<span style=""> </span>C2<span style=""> </span>C3</pre><pre>C-C-C-C<span style=""> </span>2.9288<span style=""> </span>-1.4644<span style=""> </span>0.2092<span style=""> </span>-1.6736</pre>
<pre>C-C-C-H<span style=""> </span>0.6276<span style=""> </span>1.88280<span style=""> </span>0.0<span style=""> </span>-2.5104</pre><pre>H-C-C-H<span style=""> </span>0.6276<span style=""> </span>1.8828<span style=""> </span>0.0<span style=""> </span>-2.5104</pre>
<br><br>Thankyou for any clarification.<br><br>Sulatha<br>