I need to define a dihedral angle potential which is not symmetric about 0 degree,
Something like the dihedral_style charmm and class2 but maybe more parameters.
For example, in dihedral_style charmm, a non-zero phase shifting parameter d can
make the potential is not symmetric about zero. But there are only one cosine term
in that function which is insufficient for the potential I want. But I think I could
manage to compose a complex function by using more terms.
Now what I would like to know is
1. How the torsional angle is defined in lammps.? [0,pi] or [-pi, pi] or [0,2pi]?
2. Under what circumstance, one should/could use phase shift parameter?
In CFF91, I see it is always 0. I don't know how it is used in Charmm.
3. Is there possibility to use tabulated potential for dihedrals? (Most probably is not)