Triatomic molecules benchmark handling

Handy, Carter, and Rosmus (HCR) theory, Renner-Teller effect, triatomic molecules, benchmark handling, 621-623... 

We employ the general scheme presented above as a starting point in our discussion of various approaches for handling the R-T effect in triatomic molecules. We And it reasonable to classify these approaches into three categories according to the level of sophistication at which various aspects of the problem are handled. We call them (1) minimal models (2) pragmatic models (3) benchmark treatments. The criterions for such a classification are given in Table I. 

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