Quantum Monte Carlo for chemistry, physics, the earth sciences and more...

Prof. Richard Needs (Cavendish Lab, Univ of Cambridge)

Continuum quantum Monte Carlo calculations

Abstract:
My group has developed the CASINO code [1] for performing variational and diffusion quantum Monte Carlo calculations. Fixed-node diffusion quantum Monte Carlo is the most accurate method known for calculating the energies of large many-particle quantum systems. The key ingredient is an accurate trial many-body wave function which controls the statistical efficiency and accuracy of the calculations. Accurate wave functions can be obtained by building correlation effects on top of mean field descriptions such as density functional theory or Hartree-Fock theory. About 80% of the correlation energy can typically be included by multiplying the mean-field determinants by a Jastrow factor which is small when electrons are close together and tends to unity at large separations. Such wave functions often provide an excellent description of closed shell systems but, for example, the energy released in the reaction CH2 + H2 -> CH4 is overestimated by about 5 kcal/mol (0.22 eV) because the description of CH2 is poor. This is worse than many density functionals. The wave functions of open shell systems can, however, be greatly improved by introducing multi-determinants, pairing functions, and backflow transformations, and extremely good results can be obtained. The calculations are expensive but the polynomial scaling with system size allows calculations for 1000 or more particles. The discussion of the methodology will be illustrated by recent applications to atoms, molecules and extended systems.

[1] http://www.tcm.phy.cam.ac.uk/~mdt26/casino2.html

Prof. Dario Alfè (UCL)

Recent quantum Monte Carlo calculations on weakly bound systems