title: Analytic nuclear forces and molecular properties from full configuration interaction quantum Monte Carlo.
abstract: Unbiased stochastic sampling of the one- and two-body reduced density matrices is achieved in full configuration interaction quantum Monte Carlo with the introduction of a second, "replica" ensemble of walkers, whose population evolves in imaginary time independently from the first and which entails only modest additional computational overheads. The matrices obtained from this approach are shown to be representative of full configuration-interaction quality and hence provide a realistic opportunity to achieve high-quality results for a range of properties whose operators do not necessarily commute with the Hamiltonian. A density-matrix formulated quasi-variational energy estimator having been already proposed and investigated, the present work extends the scope of the theory to take in studies of analytic nuclear forces, molecular dipole moments, and polarisabilities, with extensive comparison to exact results where possible. These new results confirm the suitability of the sampling technique and, where sufficiently large basis sets are available, achieve close agreement with experimental values, expanding the scope of the method to new areas of investigation.
authors.affiliation: The University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Thomas
authors.affiliation: The University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Daniel
authors.initials: D
authors.lastname: Opalka
authors.affiliation: The University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Catherine
authors.initials: C
authors.lastname: Overy
authors.affiliation: School of Chemistry, Cardiff University, Cardiff CF10 3AT, United Kingdom.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Knowles
authors.affiliation: The University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Ali
authors.initials: A
authors.lastname: Alavi
authors.affiliation: The University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: George H
authors.initials: GH
authors.lastname: Booth