HRAO model

Concerning the proper treatment of torsional anharmonicity, which still represents a challenging aspect for accurate thermochemical calculations of complex molecules [257-265], a hindered-rotor anharmonic oscilattor (HRAO) model has been shown to provide accurate results[62, 72, 117, 204]. The HRAO model is based on a generalization to anharmonic force fields of the hindered-rotor harmonic oscillator (HRHO) model [257] that automatically identifies internal rotation modes and rotating groups during the normal-mode vibrational analysis. 

This approach employs an effective analytical approximation of the partition function for a one-dimensional hindered internal rotation that reproduces the accurate values with a maximum error of about 2% for a number of reference systems [257]. The one-dimensional rotor treatment is generalized to give useful approximations of multidimensional rotor thermodynamic functions, and in the HRAO model, is further coupled to the simple perturbation theory (SPT) approach to the partition function for the other internal degrees of freedom [72]. 

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