Open-source software to help facilitate chemical modeling has proliferated over the past several years. Research groups can now generate and analyze detailed reaction mechanisms, systematically improve such models through numerous approaches, and systematically reduce the uncertainty in the underlying ab initio calculations to obtain sufficiently accurate models that reproduce the underlying chemistry. One implicit assumption still commonly employed in comparing experiments to ab initio calculations is that the thermophysical properties underlying temperature dependence do not introduce a significant source of error. Approximate methods are widely used except for a handful of molecules for which complete sets of molecular vibrations are known to compute the partition functions through direct summation. The non-rigid rotor anharmonic oscillator (NRRAO) model improves the ubiquitous rigid rotor harmonic oscillator (RRHO) model but is not currently widely used by computational chemists. Here an implantation of the anharmonic oscillator vibrational partition function is presented that can easily take input from vibrational perturbation theory, i.e. second-order VPT2 commonly available in numerous ab initio packages, and compute the thermophysical properties of molecular systems. Comparisons to harmonic results are given for approximately sixty molecular systems relevant in combustion modeling, and limited comparisons to benchmark full-dimensional eigenstate summations are provided where possible. These results are computed using analytic potential energy surfaces previously published for these molecules. The vibrations of non-rigid molecules that undergo large amplitude motions (LAM) are challenging as the LAM vibration is not well approximated by a harmonic oscillator. Furthermore, the cubic and quartic force constants corrections at the minima may not reasonably describe the relevant phase space for these degrees of freedom. Instead, a more general approach is presented using reduced dimensional models. In reduced dimensional models, direct eigensolutions are computed for the LAM degree(s) of freedom that replaces the harmonic or corrected anharmonic modes where necessary.

Anharmonic partition functions for molecules

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