Coordinate anharmonic cross-terms

Spectroscopists call the force constants that have all indices the same the principal force constants, while the anharmonicity associated with the principal force constants is called principal anharmonicity. The Morse and quadratic-quartic approximations treat only principal anharmonicity. However, as mentioned in Eq. [95], neglecting the cross terms between modes is a much more serious approximation in rectilinear coordinates.Explicitly including cross terms in rectilinear coordinates is expensive and cumbersome... 

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