AB Initio Liquids: Simulating Liquids Based on First Principles

Abstract

The basic ideas underlying the Car-Parrinello ab initio approach to molecular dynamics simulations are introduced. The central aspect of this technique is the first principles determination of the interactions from concurrent electronic structure calculations. Thus the usual step of introducing parameterized interaction potentials in order perform computer simulations of liquids is avoided. The currently most successful ab initio schemes treat the electrons within Hohenberg-Kohn-Sham density functional theory where generalized gradient approximations to the exchange-correlation energy are used. One example of such a first principles simulation of a liquid, the hydration of Be²⁺ in water, is presented. Secondly, an extension of the “traditional” Car-Parrinello ab initio molecular dynamics scheme to include quantum-mechanical nuclei is introduced. Within the Born-Oppenheimer separation of nuclei and electrons the nuclear degrees of freedom are quantized using Feynrnan’s path integral formulation of quantum statistical mechanics, whereas the electrons are still represented using density functional theory. This ab initio path integral technique opens the way to study quantum effects in chemically complex systems such as, e.g., an excess proton in water.