Nuclear Quantum Effects in Hydrogen-Bonded Systems: A Path Integral Molecular Dynamics Study

Abstract

Nuclei, like electrons, are quantum mechanical objects. However, their heavy mass usually results in a short de Broglie wavelength. Hence, the nuclei are usually highly localized and behave as classical particles. Nevertheless, for light nuclei and even for some heavy nuclei at low temperatures, the wave-like property becomes dominant in many cases and manifests as unexpected phenomenon. Typically, quantum fluctuations of the nuclei evince through effects like zero-point energy, tunnelling, etc. Hydrogen, the lightest nuclei in the periodic table, shows significant nuclear quantum effect (NQE). Consequently, physical properties/processes of/in H-bonded systems are affected by NQEs. For example, accurate prediction of heat capacity of water, hydrogen tunnelling affecting the reaction rates in enzyme catalysis, isotopic substitution enhances the antiferroelectric to paraelectric phase transition temperature in hydrogen-bonded ferroelectrics, and concerted tunnelling in Ih phase of ice are some of the manifestations of NQE in hydrogen-bonded system. In this thesis, using path integral molecular dynamics (PIMD) simulations, I have studied the role of NQEs in two systems: (a) a molecular crystal, terephthalic acid (TPA) and (b) an electrochemical metal/water interface, Pt(111)/water.