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Title: 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.
Authors:
; ;  [1] ;  [1] ;  [2] ;  [2] ;  [1] ;  [3] ;  [1] ;  [4]
  1. Chemistry Department, University of Cambridge, Lensfield Road, Cambridge CB2 1EW (United Kingdom)
  2. (United Kingdom)
  3. (Australia)
  4. (Germany)
Publication Date:
OSTI Identifier:
22415414
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 141; Journal Issue: 24; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; COMPUTERIZED SIMULATION; CONFIGURATION INTERACTION; DENSITY MATRIX; EXPECTATION VALUE; EXTRAPOLATION; LIMITING VALUES; MONTE CARLO METHOD; POTENTIALS; SAMPLING; SYMMETRY; VARIATIONAL METHODS; WAVE FUNCTIONS