I really appreciate the time you have spent in patiently teaching me the nuances involved in improper calculations. I have never given moltemplate a try before. I shall try it. In summary,
a) Improper style is purely dependent on the force field.
b) For LAMMPS, the central atom is the third atom (atom3 in atom1 atom2 atom3 atom4) ordering. I am assuming the same holds for improper harmonic also.
c) This is purely a doubt. In LAMMPS manual for style - harmonic/cvff, the line **"**Normally, the bonds I-J, I-K, I-L would exist for an improper to be defined between the 4 atoms." sort of gives the impression that “i” (or atom1 in the above order) is the first atom. Probably this is what led me to all the confusion. Since atom3 is the central atom, I shall redefine the improper part of datafile in that way.
Thanks a lot again for the help.
Vaidyanathan M S