Coupled Anharmonic Thermochemistry from Stratified Monte-Carlo Integration

This study presents CIMCI, a new semi-classical method for handling fully coupled anharmonicity in gas-phase thermodynamics that promises to be black-boxable, to be
applicable for all kinds of anharmonicity, and to scale better at higher dimensionality than other methods for handling gas-phase molecular anharmonicity. The method
does so by using automatically and recursively stratified, simultaneous Monte Carlo integration of multiple functions. For the small systems analyzed by this study, the
method’s anharmonic corrections match reference data better than those of other blackbox anharmonic methods, e.g. VPT2. This holds even when sampling with CIMCI
is done with primitive force fields, e.g. UFF, while the competing methods are used with proper, comprehensive potentials, e.g. the M06-2X meta-hybrid DFT functional.
With further refinements in Monte Carlo sampling efficiency, in the quality of fast potentials practical for Monte Carlo sampling, and in automatic detection of which stereoisomers should be included during sampling, CIMCI has the potential to be the ideal anharmonic treatment for larger molecules where the large number of conformers
and the high dimensionality of coupled torsions present major difficulties for other, existing treatments for anharmonicity.