Frozen electron gas models for molecular dynamics of liquids

• Main Author: Barker, D. R.
• Published: University of Cambridge 2002
• Subjects: 530.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596361

Methods for improving the size of accurate Kohn-Sham (KS) DFT calculations have often focused on embedding the KS calculation within a region of fixed electronic density. This thesis examines the potential for a similar type of approach within Car-Parrinello, <i>ab-initio</i>, Molecular Dynamics (CPMD). A frozen electron density (FED) model is incorporated into a KS-DFT based simulations of liquid water. Interactions between FED and KS species are determined using total energy expressions taken from the study of embedded systems. The initial step during this development was a study of the FED electron gas model proposed by Gordon and Kim for rare gas systems, using a modified version of the DL_POLY MD code. The adapted code was applied to investigate various options for extending this scheme to inter-molecular interactions in liquid water, including assessment of the suitability of a number of kinetic energy (KE) functionals. The optimisation of electron density for a FED model of water was explored semi-empirically, based on adaptation of the fixed molecular electron density to the condensed phase environment. Consistent with experience from force field methods, enhancement of the molecular dipole moment proved to be necessary to reproduce the properties of the liquid. Using the TF, PW01 and LLP KE functionals, models giving good agreement with experimental results were obtained. These each carried a dipole moment of 2.95 D, the same as has been observed by recent <i>ab-initio</i> MD studies based on fully self-consistent KS methods. Finally the CPMD code was adapted to incorporate these models into mixed KS/FED simulations, in which both molecular types occupied the same simulation cell. All MD runs showed energetic stability and good overall structure. However, more acceptable reproduction of the inter-molecular interactions of a fully KS-DFT calculation were obtained with the PW91 and LLP based models, than with that based on the TF functional. This is in agreement with similar calculations reported for the gas phase dimer.