<div>Dear GMX-users,</div><div><br></div><div>I found a question in 2009 asking which format of improper does OPLS dihedral gromacs use. I have the same question, is it periodic or harmonic?</div><div>If it is periodic, why in its ffbonded.itp file: <span class="Apple-style-span" style="font-family: monospace; white-space: pre; font-size: medium; ">#define improper_O_C_X_Y 180.0 43.93200 2 what do they mean?</span></div>
<div><font class="Apple-style-span" face="monospace" size="3"><span class="Apple-style-span" style="white-space: pre;">Also, when I plug in tinker's improper dihedrals(which should be harmonic) into gromacs, the results matches well whith each other(tinker and gromacs). Does it suggest that gromacs uses a periodic dihedral?</span></font></div>
<div><span class="Apple-style-span" style="font-family: monospace; white-space: pre; font-size: medium; "><br></span></div><div><span class="Apple-style-span" style="font-family: monospace; white-space: pre; font-size: medium; ">Thanks, here is the part I quoted from ffbonded.itp</span></div>
<div><br></div><div>[ dihedraltypes ]</div><div>; Improper OPLS dihedrals to keep groups planar.</div><div>; (OPLS doesnt use impropers for chiral atoms).</div><div>; Since these functions are periodic of the form 1-cos(2*x), they are actually</div>
<div>; implemented as proper dihedrals [1+cos(2*x+180)] for the moment,</div><div>; to keep things compatible.</div><div>; The defines are used in ffoplsaa.rtp or directly in your .top file.</div><div><br></div><div>--jia</div>