[gmx-users] Question regarding dihedral angle parameters

[Upendra N]

Dear Gromacs users,

I am a beginner in gromacs. I was trying to understand how dihedral terms
are used and I noticed that for Amber99 ff the ffbonded.itp files have
mutliple dihedral parameters for the same dihedral type, each with
different force constant, multiplicity and phase angle

For instance,

 CT  OS  CT  OS    9       0.0      0.41840     3  ; Junmei et al, 1999
 CT  OS  CT  OS    9     180.0      3.55640     2  ; Junmei et al, 1999
 CT  OS  CT  OS    9     180.0      5.64840     1  ; Junmei et al, 1999

I am wondering what determines which of this is assigned to a given
dihedral.

Also, why is the phase shift different as I understand that phase shift
determines where is the minima of the energy in the energy vs torsion plot.

I have tried looking for the reason but somehow have not found a
satisfactory answer. I would really appreciate if someone can clarify this
to me.

Thank you.

Upendra N