Comprehensive classical analysis of partition function and some observables for weakly interacting polyatomic dimers

This paper presents the systematic classical consideration of a statistical averaging procedure that permits the calculation of partition function, equilibrium constant, and some observables for polyatomic dimers composed of weakly interacting rigid monomers. It was shown that the number of independent internal coordinates in a body-fixed frame is a crucial parameter that largely determines the temperature dependence of the partition function irrespective of the kinematic coupling within various degrees of freedom. The kinetic energy was derived for the molecular pair of arbitrary complexity in the body-fixed frame. Rigorous expression was obtained for the partition function over a pre-selected domain in the phase space. A similar expression was applicable to perform statistical averaging of some observables. Taking a linear molecule-atom as an example, it was shown how the suggested general approach permits the calculation of the equilibrium constant for true bound dimer formation or zeroth spectral moment of a collision-induced absorption band.