Pitzer-Gwinn approximation

Pitzer-Gwinn approximation, coupled with constraints on the range of bond-lengths to maintain undissociated fragments. 

Statistical Thermodynamics of Bond Torsional Modes. Tests of Separable, Almost-Separable, and Improved Pitzer-Gwinn Approximations. 

For heavy asymmetric rotors the torsional levels may be sufficiently low for thermodynamic properties to be estimated by using the classical approximation. Gwinn and Pitzer have expressed the classical approximation to the complete internal partition function as... 

Pitzer and Gwinn have considered calculations for molecules having potential energy cross-terms between vibrational and internal rotational motion. Here is not, even to a first approximation, independent... 

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