An approximation for hindered rotor state energies

A method suggested by Troe [J. Chem. Phys. 66 (1977) 4758] for estimating hindered internal rotation state energies is first evaluated and then extended by including anharmonic terms and introducing a smooth switching function. Comparisons between calculated thermodynamic functions and exact results [J. Chem. Phys. 10 (1942) 428] show that the extended method is greatly improved and performs at a level comparable to other approximate methods, but without their drawbacks. The extended method is very useful for calculating the densities of states and partition functions needed for reaction rate theories. (C) 2003 Elsevier B.V. All rights reserved.