Approximate factorization of molecular potential surfaces II. Internal rotors

Scaling models for the polyatomic potential energy surface occupy a ground intermediate between ab initio surfaces and random matrix models. They are useful for the study of vibrational energy redistribution (IVR) in large mo lecules, and have been shown to reproduce many features of more accurate sp ectroscopically fitted or ab initio potential surfaces. A previous analysis of potential constants and coupling matrix elements [J. Chem. Phys. 106 (1 977) 5874] considered modes that maintain vibrational character up to the d issociation limit. Here we discuss factorization and scaling properties of the vibrational Hamiltonian in the presence of an internal rotor. We find t hat the "rotor effect" is most pronounced for delocalized skeletal vibratio ns, potentially resulting in increased NR of "bath" states compared to "bri ght" states. For localized vibrational modes that involve atomic displaceme nts near the rotor, the local density of states is enhanced, while for more remote localized modes, the effect is negligible.