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Using Density Matrix Quantum Monte Carlo for Calculating Exact-on-Average Energies for ab Initio Hamiltonians in a Finite Basis Set
Journal article   Open access   Peer reviewed

Using Density Matrix Quantum Monte Carlo for Calculating Exact-on-Average Energies for ab Initio Hamiltonians in a Finite Basis Set

Hayley R Petras, Sai Kumar Ramadugu, Fionn D Malone and James J Shepherd
Journal of chemical theory and computation, Vol.16(2), pp.1029-1038
02/11/2020
DOI: 10.1021/acs.jctc.9b01080
PMID: 31944692
url
https://www.osti.gov/servlets/purl/1631089View
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Abstract

We here apply the recently developed initiator density matrix quantum Monte Carlo (i-DMQMC) to a variety of atoms and molecules in vacuum. i-DMQMC samples the exact density matrix of a Hamiltonian at finite temperature and combines the accuracy of full configuration interaction quantum Monte Carlo (FCIQMC)-full configuration interaction (FCI) or exact energies in a finite basis set-with finite temperature. In order to explore the applicability of i-DMQMC for molecular systems, we choose to study a recently developed test set by Rubenstein and co-workers: Be, H O, and H at near-equilibrium and stretched geometries. We find that, for Be and H O, i-DMQMC delivers energies with submillihartree accuracy when compared with finite temperature FCI. For H O and both geometries of H , we examine the difference between FT-AFQMC and i-DMQMC, which, in turn, estimates the difference in canonical versus grand canonical energies. We close with two discussions: one of simulation settings (initiator error, the interaction picture, and different basis sets), and another of energy difference calculations in the form of specific heat capacity and ionization potential calculations.

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