Assessing the environmental influence on ‘local-monomer’ vibrational spectra via many-body potentials

The ‘local-monomer’ method serves as both a conceptual framework and numerically useful computational approach for studying the vibrational spectra of molecular clusters, liquids, and molecular solids. Within such a framework, internally coupled monomer vibrations oscillate in the full electronic response of surrounding monomers. Many-body potentials afford the possibility of exploring the roles played by components of this response – including n-body terms, permanent and induced electrostatics, and dispersion – in influencing the resulting spectra. In this investigation, the MB-pol water potential is used to decompose anharmonic couplings in local-monomer spectra of water clusters, (H2O)6 to (H2O)102. All potential-energy components were found to be critical for single-mode harmonic and anharmonic behaviour. Two-mode anharmonic couplings, however, were largely captured by one-body (monomer) terms in the potential, with some transitions reliant on many-body polarisation in the stronger collective environments of larger clusters. The remnant three-mode anharmonic coupling effects were mostly captured by one-body potential terms. Encouragingly, most of these truncated potentials still included resonant vibrational effects that are reliant on mode coupling and anharmonicity. These outcomes suggest the possible development of reduced-cost anharmonic simulation methods and also provide insight into the nature of the evidently local environmental response encoded in water clusters.