Type 9 dihedral with multiple entries in [dihedraltypes]

GROMACS version: 2021.2
GROMACS modification: No

Hi all,

Hope someone can help me clarify this point:

I have the type 9 dihedral interaction in my forcefield files, and hope to further tune these values. The entries are:

; i j k l func phi0 kphi mult
CG321 CG321 NS1 CG321 9 0.00 6.9 1 ;
CG321 CG321 NS1 CG321 9 0.00 1.2 2 ;
CG321 CG321 NS1 CG321 9 0.00 3.1 3 ;

The I j k l atoms are exactly the same and the multiplicity values are 1 and 2. The user
s manual says “Type 9 allows multiple potential functions to be applied automatically to a single dihedral in the [ dihedral ] section when multiple parameters are defined for the same atomtypes in the [ dihedraltypes ] section.”

I do not quite understand this statement. I have one line in the [dihedrals] section of my .top file, which exactly hits the “CG321 CG321 NS1 CG321” dihedral.

Does it mean that the actual ff is the sum of the terms of
V = Kphi(1+cos(n*phi-phi0))

i.e. V_dih = 6.9*(1+cos(1 * phi - 0.0)) + 1.2*(1+cos(2 phi -0.0 ) +3.1(1+cos(3 *phi -0.0 ), ???

Hope someone can help me confirm this.

You are correct. CHARMM allows multiple cosine terms to be summed to give the total dihedral energy contribution.

Thank you professor!