Rovibrator nonrigid

Purely quantum studies of the fully coupled anharmonic (and sometimes nonrigid) rovibrational state densities have also been obtained with a variety of methods. The simplest to implement are spectroscopic perturbation theory based studies [121, 122, 124]. Related semiclassical perturbation treatments have been described by Miller and coworkers [172-174]. Vibrational self-consistent field (SCF) plus configuration interaction (Cl) calculations [175, 176] provide another useful alternative, for which interesting illustrative results have been presented by Christoffel and Bowman for the H + CO2 reaction [123] and by Isaacson for the H2 + OH reaction [121]. The MULTIMODE code provides a general procedure for implementing such SCF-CI calculations [177]. Numerous studies of the state densities for triatomic molecules have also been presented. 

Finally, in physical situations characterized by potential energy functions intermediate between purely rigid and nonrigid rovibrators, one should consider more complex algebraic treatments in which both U(3) and 0(4) invariant operators are included. Consequently, the Hamiltonian operator can no longer be diagonal in the chosen algebraic basis (related to either one or the other of the two dynamical symmetries). However, matrix elements for any operator of interest have already been explicitly computed in analytical form [35]. 

Figure 2.5 Energy-level diagram of the nonrigid three-dimensional rovibrator [Eq. (2.105)]. Here N = 3 and e, k, k >0.

Tennyson, J. and Sutcliffe, B.T (1983) Variationally exact rovibrational spectra of nonrigid triatomics the HeHF van der Waals molecule. /. Chem. Phys., 79, 43. 

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