State-of-the-art molecular applications of full configuration interaction quantum Monte Carlo

Full configuration interaction quantum Monte Carlo (FCIQMC)$^1$ and its initiator adaptation (\emph{i}-FCIQMC)$^2$ provide, in principle, exact (FCI) energies \emph{via} a population dynamics algorithm of an ensemble of discrete, signed walkers in Slater-determinant space. We demonstrate that a novel choice of reference state has the potential to widen the scope of this already versatile method, and corroborate the finding that an extension of the algorithm to allow non-integer walkers can yield significantly reduced stochastic error without a commensurate increase in computational cost$^3$. New applications of FCIQMC to transition-metal systems of general and biological interest are presented, many of which have, to date, posed serious challenges for traditional quantum chemical methods$^4$$^5$. $^1$ G. H. Booth, A. J. W. Thom, and A. Alavi, J. Chem. Phys., 131, 054106 (2009) $^2$ D. M. Cleland, G. H. Booth, and A. Alavi, J. Chem. Phys., 132, 041103 (2010) $^3$ F. R. Petruzielo, A. A. Holmes, H. J. Changlani, M. P. Nightingale and C. J. Umrigar, arXiv:1207.6138 $^4$ N. B. Balabanov and K. A. Peterson, J. Chem. Phys., 125, 074110 (2006) $^5$ C. J. Cramer, M. Wloch, P. Piecuch, C. Puzzarini and L. Gagliardi, J. Phys. Chem. A, 110, 1991 (2006)