Calculation of rotational states of methane molecules in a square lattice

Methane molecules in a square lattice perform hindered rotation due to the molecular interaction. First, the one-body term in the interaction gives th e new pattern of rotational energy level scheme, In the case of strong inte raction, this scheme becomes similar to tunneling levels calculated from th e pocket state formalism, Secondly, the two-body term produces a potential with lower symmetry. We investigate the case of a ferro-orientational phase . Four levels remain in the lowest part of rotational energy spectrum under the field strength estimated from the sum of atom-atom pair potentials. Th is small number of levels approaches to more realistic systems. (C) 1999 El sevier Science Ltd. All rights reserved.