Triatomic molecule nonrigidity

Jensen, P. (1983), The Nonrigid Bender Hamiltonian for Calculating the Rotation-Vibration Energy Levels of a Triatomic Molecule, Comp. Phys. Rep. 1,1. 

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. 

The abscissas of the correlation diagrams can be quantified. The quantification is thus far only an ex post facto device with power to classify but not yet to predict. It does not yet have a microscopic interpretation. Nevertheless it satisfies the needs of a theory of the equilibrium behavior of melting and freezing of clusters. Only the existence and not the definition of the nonrigidity parameter or the details of its origins is used in stage 2. However, we make a brief aside here to explain the definition in order to clarify just what information it carries. To understand the order parameter y (not to be confused with the surface tension), it is useful to examine how nonrigidity is traditionally characterized in diatomic and linear, particularly triatomic molecules. For... 

Other Hamiltonians, which also do not require knowledge of the full three-dimensional potential curve for triatomic molecules but use only their bending part in addition to various sections of the stretching potential, are the so-called rigid bender,semi-rigid, and nonrigid bender Hamiltonians. ... 

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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