Dear Dr. Kohlmeyer,

My sincerest apologies for not having introduced myself previously. My name is Felipe Perez, a current PhD student in Petroleum Engineering at the University of Oklahoma. I am writing because I have a question regarding the implementation of pcff force field in LAMMPS using msi2lmp.exe. In the pcff.frc file, the reported expressions for torsion_3 and torsion_1 are, respectively,

E = SUM(n=1,3) { V(n) * [ 1 + cos(n*Phi - Phi0(n)) ] }

and

E = Kphi * [ 1 + cos(n*Phi - Phi0) ].

However, class 2 force fields (their implementation in LAMMPS) have the form

E = SUM(n=1,3) { K(n) * [ 1 - cos(n*Phi - Phi0(n)) ] }

(Sun, 1995 presents this same form except that Phi0(n) = 0 for all n).

If the expression reported in pcff.frc is right, I have to add 180.0° to all Phi0 parameters so that once I input them in LAMMPS, the expression becomes (only the trigonometric function)

cos(n*Phi - Phi0(n) + 180°) = cos(n*Phi - Phi0(n))*cos(180°) - sin(n*Phi - Phi0(n))*sin(180°)

= - cos(n*Phi - Phi0(n)),

so that I recover the equation for which the equilibrium angles are given.

Is this correct?

Thanks in advance for your kind attention. Again, my apologies if this is happens to be a silly question.

Best regards,

Felipe Perez.