The starting point for understanding the properties of atoms, molecules and solids is the many-electron Schrodinger equation. Solving this equation is a theoretically challenging and computationally demanding problem, and one of the most promising approaches is the use of statistical techniques known as quantum Monte Carlo (QMC) methods. QMC methods come in variety of flavours, and we use the variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) methods. These are very promising for applications to condensed matter because (i) they give an accurate description of electron correlation effects and (ii) the computational cost scales well with system size, normally something like the qsuare or the cube of the number of atoms. VMC and DMC calculations have been applied to systems containing over 1000 electrons.
The standard approach to electronic structure computation has been the use of mean-field methods such as Hartree-Fock and local density functional theory. These methods have been outstandingly successful, but the mean-field picture is not always accurate enough to give a proper physical understanding. Our QMC techniques use the mean-field approximation as a starting point for the calculations, but they also include an explicit and accurate description of correlation effects.
We perform our calculations using the CASINO code. We have applied QMC techniques to calculating ground and excited state energies of solids and molecules, including studies of defects in solids, solid surfaces, relativistic effects, phase transitions, and calculations aimed at developing new density functionals.