“…Many attempts have been made to improve on the separable approximation by looking at the specific coupling between different modes. For instance, the coupling of bends to stretches has been studied and empirical models describing this coupling have been constructed, − the role of stretch–stretch coupling has been investigated in triatomic systems, , numerous methods for treating torsional motions have been developed, ,,− and Monte Carlo integration has been applied to calculation of quantum vibrational states using a spectroscopic (e.g., Dunham) expansion, which includes some anharmonic terms. − It is also worth noting the recent semiempirical work of Schmatz as well as the thermodynamic method, which relies on experimental data, , the density correlation function method of Jeffreys et al, , and the use of path-integral methods to calculate the quantum partition function directly. , The accurate inclusion of the coupling terms, however, remains an open issue.…”