Dear gmx-users,

I am confused about the exact formula for improper OPLS dihedrals in gromacs.
Is it periodic, e.g. V[1-cos(2x)], or harmonic, e.g. [V*(x-x0)^2]?
In ffoplsaa.rtp, it says in the Col 4 in the following the type of improper dihedrals to be 1,
which I believe is a harmonic function. The gromacs manual says the same.

[ bondedtypes ]
; Col 1: Type of bond 
; Col 2: Type of angles
; Col 3: Type of proper dihedrals
; Col 4: Type of improper dihedrals
; Col 5: Generate all dihedrals if 1, only heavy atoms of 0.
; Col 6: Number of excluded neighbors for nonbonded interactions
; Col 7: Generate 1,4 interactions between pairs of hydrogens if 1
; Col 8: Remove propers over the same bond as an improper if it is 1
; bonds  angles  dihedrals  impropers all_dihedrals nrexcl HH14 RemoveDih
     1       1          3          1	    1         3      1     0

Meanwhile, in ffoplsaabon.itp, it appears that the periodic function, not harmonic, is used for
improper dihedrals, as shown in the following:

[ 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), the 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 ile.

Which one is correct? Periodic or harmonic? And what does each number in improper dihedral
parameters mean? For example, what do 180.0, 43.93200, and 2 mean in the following improper
dihedral parameters?

#define improper_O_C_X_Y        180.0     43.93200   2

Many thanks in advance for your help!

Best,
Sung