A general rotation-contortion Hamiltonian with structure relaxation: Application to the processing internal rotor model

A general computer program has been written to compute rotation-contortion energy levels, using the semirigid-bender structure-relaxation Hamiltonian, for any molecule having one large amplitude (contortional) degree of freedom. In an application it is necessary to input the contortional potential energy function and the expressions for the molecule fixed xyz coordinates of the nuclei as functions of the contortional coordinate. The contortional coordinate can be of any type, such as an internal rotation, an inversion, or a bend, but the boundary conditions in the numerical integration part of the program must be chosen appropriately; these boundary conditions are in a separate subroutine. The code is applied to the model of a tilted and precessing internally rotating CH3+ group in protonated methane, CH5+. From the results we can determine quantitatively the effect of the approximations present in the Hamiltonian of X-Q. Tan and D. W. Pratt [J. Chem. Phys. 100, 7061-7067 (1994)] when the angle of tilt of the internal rotation axis becomes large. (C) 1997 Academic Press.