[gmx-users] improper OPLS dihedrals in gromacs

[mu xiaojia]

Dear GMX-users,

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?
If it is periodic, why in its ffbonded.itp file: #define improper_O_C_X_Y
180.0 43.93200 2 what do they mean?
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?

Thanks, here is the part I quoted from ffbonded.itp

[ dihedraltypes ]
; Improper OPLS dihedrals to keep groups planar.
; (OPLS doesnt use impropers for chiral atoms).
; Since these functions are periodic of the form 1-cos(2*x), they are
actually
; implemented as proper dihedrals [1+cos(2*x+180)] for the moment,
; to keep things compatible.
; The defines are used in ffoplsaa.rtp or directly in your .top file.

--jia
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