Analytic nuclear forces and molecular properties from full configuration interaction quantum Monte Carlo

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.Comment: 11 page

Does Osmotic Stress Affect Natural Product Expression in Fungi?

Acknowledgments: Russell Kerr acknowledges the assistance of Nadia Prigoda-Lee, Marius Grote, Kate McQuillan and Stephanie Duffy, and generous financial support from NSERC, the Canada Research Chair program, the Jeanne and Jean-Louis Lévesque Foundation and the Atlantic Canada Opportunities Agency. Ka-Lai Pang thanks the president of National Taiwan Ocean University, Ching-Fong Chang, for a special fund to attend the workshop held in Charlottetown, Canada in 2014 where this work was discussed. Rob Capon and Zhuo Shang acknowledge support from the University of Queensland, and the UQ Institute for Molecular Bioscience. Zhuo Shang acknowledges the provision of an International Postgraduate Research Scholarship (IPRS) and a Centennial Scholarship by the University of Queensland. Catherine Roullier acknowledges the assistance of Marie-Claude Boumard and Thibaut Robiou du Pont, and support from Region Pays de la Loire, FrancePeer reviewedPublisher PD

Unbiased reduced density matrices and electronic properties from full configuration interaction quantum Monte Carlo.

Properties that are necessarily formulated within pure (symmetric) expectation values are difficult to calculate for projector quantum Monte Carlo approaches, but are critical in order to compute many of the important observable properties of electronic systems. Here, we investigate an approach for the sampling of unbiased reduced density matrices within the full configuration interaction quantum Monte Carlo dynamic, which requires only small computational overheads. This is achieved via an independent replica population of walkers in the dynamic, sampled alongside the original population. The resulting reduced density matrices are free from systematic error (beyond those present via constraints on the dynamic itself) and can be used to compute a variety of expectation values and properties, with rapid convergence to an exact limit. A quasi-variational energy estimate derived from these density matrices is proposed as an accurate alternative to the projected estimator for multiconfigurational wavefunctions, while its variational property could potentially lend itself to accurate extrapolation approaches in larger systems