1,589 research outputs found

    Accurate exchange-correlation energies for the warm dense electron gas

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    Density matrix quantum Monte Carlo (DMQMC) is used to sample exact-on-average NN-body density matrices for uniform electron gas systems of up to 10124^{124} matrix elements via a stochastic solution of the Bloch equation. The results of these calculations resolve a current debate over the accuracy of the data used to parametrize finite-temperature density functionals. Exchange-correlation energies calculated using the real-space restricted path-integral formalism and the kk-space configuration path-integral formalism disagree by up to ∼\sim1010\% at certain reduced temperatures T/TF≤0.5T/T_F \le 0.5 and densities rs≤1r_s \le 1. Our calculations confirm the accuracy of the configuration path-integral Monte Carlo results available at high density and bridge the gap to lower densities, providing trustworthy data in the regime typical of planetary interiors and solids subject to laser irradiation. We demonstrate that DMQMC can calculate free energies directly and present exact free energies for T/TF≥1T/T_F \ge 1 and rs≤2r_s \le 2.Comment: Accepted version: added free energy data and restructured text. Now includes supplementary materia

    An excited-state approach within full configuration interaction quantum Monte Carlo.

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    We present a new approach to calculate excited states with the full configuration interaction quantum Monte Carlo (FCIQMC) method. The approach uses a Gram-Schmidt procedure, instantaneously applied to the stochastically evolving distributions of walkers, to orthogonalize higher energy states against lower energy ones. It can thus be used to study several of the lowest-energy states of a system within the same symmetry. This additional step is particularly simple and computationally inexpensive, requiring only a small change to the underlying FCIQMC algorithm. No trial wave functions or partitioning of the space is needed. The approach should allow excited states to be studied for systems similar to those accessible to the ground-state method due to a comparable computational cost. As a first application, we consider the carbon dimer in basis sets up to quadruple-zeta quality and compare to existing results where available.N.S.B. gratefully acknowledges Trinity College, Cambridge for funding. G.H.B. gratefully acknowledges the Royal Society for funding via a university research fellowship. This work has been supported by the EPSRC under grant no. EP/J003867/1.This is the accepted manuscript. The final version is available at http://scitation.aip.org/content/aip/journal/jcp/143/13/10.1063/1.4932595

    Open-source development experiences in scientific software: the HANDE quantum Monte Carlo project

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    The HANDE quantum Monte Carlo project offers accessible stochastic algorithms for general use for scientists in the field of quantum chemistry. HANDE is an ambitious and general high-performance code developed by a geographically-dispersed team with a variety of backgrounds in computational science. In the course of preparing a public, open-source release, we have taken this opportunity to step back and look at what we have done and what we hope to do in the future. We pay particular attention to development processes, the approach taken to train students joining the project, and how a flat hierarchical structure aids communicationComment: 6 pages. Submission to WSSSPE
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