Ab Initio Quantum Approaches to the IR Spectroscopy of Water and Hydrates

This Perspective highlights progress in ab initio quantum approaches to IR spectroscopy of water and hydrates. Here, "ab initio" refers to many-body potentials and dipole moment surfaces for flexible water and hydrates. Specifically, these are mathematical representations of two-body and three-body interactions based on permutationally invariant fitting of tens of thousands of ab initio electronic energies, a spectroscopically accurate one-body monomer potential, and four- and higher-body interactions described by the long-range interactions incorporated into, for example, the TTM3-F family of potentials. There are currently two such potentials of this type, denoted WHBB and MB-pol, which are being used in expanding applications. Here, the focus is on infrared spectroscopy, using the WHBB potential and dipole moment surface, with an embedded, local monomer quantum method to obtain vibrational energies and dipole transition moments. Comparisons are also made with the popular q-TIP4P/F potential. Brief mention is made of an application to small HCl-H2O clusters.