Vibrational anharmonicity contributes significantly to the energies and entropies coming from atomistic simulation. In order to capture these effects, quantum chemical methods have been proposed to go beyond the harmonic approximation and describe the physically important anharmonicities and anharmonic couplings between modes.<sup>[1,2]</sup> Due to the immense expense of computing fully anharmonic energy levels, however, approximations must be made to the vibrational wave functions. Our approach to address these challenges provides a tunable windowing strategy to optimize of local modes, and perform vibrational configuration interaction within these windows. This allows the higher-order mode-mode couplings in the vibrational potential energy surfaces to be neglected, ultimately resulting in dramatically lower costs. To demonstrate the applicability and transferability of the proposed method, we will show a series of benchmarks for the vibrational entropy and anharmonic vibrational spectra of organic molecules and protonated water clusters, and that the calculated results are in a good agreement with the experimental spectra. Finally, comparisons of approximated total vibrational energies and entropies to the harmonic and exact limits will numerically validate that high accuracy can be maintained, even with substantially reduced computational costs.<br/><br/>

Useful relationships between localized vibrational modes and anharmonic entropy

2019 Fall National Meeting

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