A Distributed Hyperpolarizability Model for Liquid Water

In a recent work a novel, molecular dynamics (MD) based, theory was presented that provides a direct means to calculate the third order contributions to sum frequency vibrational spectroscopy (SFVS) at charged interfaces.[1] This contribution is described by a time correlation function (TCF) of the system's total hyperpolarizability (β) and dipole. A system's hyperpolarizability[2, 3, 4, 5] is difficult to calculate on the fly – no appropriate non-electronic structure (ES) techniques exist. In this work, such a model is developed for liquid water based on a formalism proposed previously.[1] The model is parametrized from and tested against, ab initio ES calculations. It was found that it is possible to reproduce the hyperpolarizability tensor of a gas phase water molecule and a water dimer accurately. Also, most relevant components of the hyperpolarizability derivative tensor of both the water and water dimer were also accurately fit, however, some elements were not able to be simultaneously reproduced. The model was also tested to assess whether or not it could reproduce the three non-zero condensed phase water hyperpolarizability tensor components. It was found that the model could capture two of the three relevant components.