Ab initio molecular dynamics simulation of liquid water by quantum Monte Carlo

Abstract

Although liquid water is ubiquitous in chemical reactions at roots of life and climate on the earth, the prediction of its properties by high-level ab initio molecular dynamics simulations still represents a formidable task for quantum chemistry. In this article we present a room temperature simulation of liquid water based on the potential energy surface obtained by a many-body wave function through quantum Monte Carlo (QMC) methods. The simulated properties are in good agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function, at variance of previous Density Functional Theory attempts. Given the excellent performances of QMC on large scale supercomputers, this work opens new perspectives for predictive and reliable ab-initio simulations of complex chemical systems. PACS numbers:The simulation by first principles of liquid water, the key element of human life and biological processes, has been a dream for several decades after the foundation of Density Functional theory (DFT), even within the restriction of the Born-Oppenheimer approximation for the heavy nuclei. Realistic simulations are particular important because, at the experimental level, it is not possible to clarify completely what are the relationships between the so many different and rich phases of water and the physical interactions between water molecules, determined by hydrogen bonding and weak longrange van der Waals (vdW) interactions. Moreover water is involved in many biological and chemical processes, and first principle simulations are useful to investigate and rationalize such important mechanisms.The first attempted simulations date back to the pioneer works by Car and Parrinello 1-3 , within an efficient ab-initio molecular dynamics (AIMD) based on DFT. The comparison with the experiments, at that time available, provided a pretty good agreement with the oxygen-oxygen (O-O) radial distribution function (RDF), as far as the positions of the peaks were concerned, but the overall shape given by the simulation was overstructured. After these first studies, many other works reporting standard DFT-based simulations have been published, but the agreement with the experimental data is still not satisfactory on many aspects. The equilibrium density at ambient pressure (1 atm ∼ 10 −4 GPa), is far to be consistent with the expected one (1 gr/cm 3 ) though recent DFT functionals including van der Waals substantially reduce this discrepancy 4 . The simulated diffusion 5 is much lower than what is expected from experiments 6 , and, at least in some functionals (namely, PBE and BLYP), the solidification of water occurs at a temperature which is unrealistically large (∼ 410 K), so that some of the present DFT simulations of liquid water should be considered supercooled metastable phases 6,7 .The DFT results (about which we provide a brief summary in Tab. I) appear to be substantially influenced by the choice of the functional 5,8 , but also, within a ...

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