Ab Initio Path Integral Molecular Dynamics Simulation of Liquid Water (WAPIMD)

So Hirata, University of Illinois at Urbana-Champaign

Usage Details

Ab initio Born-Oppenheimer path integral and thermostatted ring polymer molecular dynamics (PIMD and TRPMD) ,  simulations of the structural and dynamical properties of liquid water, respectively, will be performed using the atomic forces obtained from the embedded-fragment many-body perturbation or coupled-cluster method. Previous classical simulations5 have shown that such treatment of water at the level of second-order many-body perturbation (MP2) theory reproduces the radial distribution function, self-diffusion coefficient, and infrared (IR) and Raman spectra accurately. However, classical simulations do not take into account the nuclear quantum effects, and as a result, the phase diagram and the vibrational spectra are still in considerable error. For this reason, path-integral quantum simulations are important for further improvement.  Both static (RPMD, imaginary-time) and dynamical (RPMD, real-time) simulations will be run under a Langevin thermostat which will be applied to all the internal modes of the ring polymer which represents a necklace in the extended phase space of classical copies of the original quantum system.8 Such approach has proven to be superior over standard thermostatting schemes, both from a convergence and from an accuracy point of view.

The main goal of the present study is to reproduce the main experimentally observed properties of liquid water (such as oxygen-oxygen radial distribution, self-diffusion coefficient, IR and Raman spectra, phase diagram (melting point et c.)) and to develop a framework for accurate liquid water simulations from first principles which could be used by the community in further studies.