Predictions of properties of matter require force fields, i.e., potential energy surfaces (PESs) which determine nuclear motions. Physics-based PESs fitted to ab initio quantum mechanical calculations (APES) are becoming the potentials of choice. Among ab initio methods, symmetry-adapted perturbation theory (SAPT) [WIRES 2, 254 '12] is particularly well suited for such calculations due to its seamless connection to asymptotic interaction energies. Recently, generation of APES became as simple as running standard electronic structure programs thanks to the automation of this process in the autoPES package [JTCT 12, 5895 '16]. In a sufficiently high order, SAPT becomes arbitrarily close to full configuration interaction results [Struct. & Bond. 116, 43 '05]. In practice, SAPT gives results of similar quality as CCSD(T) at similar costs. However, a version of SAPT based on density-functional theory, SAPT(DFT), gives results of such quality at DFT costs [JCP 145, 124105 '16]. APESs predict interaction energies that are much more accurate than either those from empirical PESs or from DFT on-the-fly calculations. At the same time, the resources needed for molecular dynamics simulations with APES are only a few times larger than in the case of empirical PESs. Several applications of APES will be discussed. For water and water-methane, nearly exact quantum nuclear dynamics calculations based on recently developed APES agree with experimental spectra as well or better than any published predictions. With the methane-methane PES developed in the same work, it will possible to investigate methane clathrates more accurately than ever before. Very good agreement with experiment has been reached for the water second and third virial coefficients, quantities that are accurately known from experiments but could not be reproduced to within experimental uncertainties by earlier PESs. Results will be presented for a set of organic molecules built of about 20 atoms each: a project aimed at design of novel cocrystals. Ongoing attempts at growing such cocrystals use results of crystal structure predictions from molecular simulations based on APESs.

Nuclear dynamics with accurate force fields from quantum mechanics

2019 Spring National Meeting

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