The form of OPLS_DIHEDRAL in the manual (eqn 3.52) and in the code has a constant term: (1/2)p0 and has three 1 +/- cos() terms with periodicities 1,2,3. All of the OPLS dihedral forms I see in the literature have no constant term and either the 3 cos terms or 4. Why the difference? If implementing OPLS FF / molecules, does anyone use the OPLS_DIHEDRAL form or do they use, e.g. the FOURIER_SERIES_DIHEDRAL (with p4 = p5 = 0 and usually p3 = 0 since only a few dihedrals have been published using the periodicity=4 term) form? Am I missing something?

Thanks for any help.