Full Configuration Interaction Quantum Monte Carlo (FCIQMC) has been effectively applied to very large configuration interaction (CI) problems and was recently adapted for use as an active space solver and combined with orbital optimization. In this work, we detail an approach within FCIQMC to allow for efficient sampling of fully internally contracted multireference perturbation theories within the same stochastic framework. Schemes are described to allow for the close control over the resolution of stochastic sampling of the effective higher-body intermediates within the active space. It is found that while complete active space second-order perturbation theory seems less amenable to a stochastic reformulation, strongly contracted N-Electron Valence second-order Perturbation Theory (NEVPT2) is far more stable, requiring a similar number of walkers to converge the sc-NEVPT2 expectation values as to converge the underlying CI problem. We demonstrate the application of the stochastic approach to the computation of sc-NEVPT2 within a (24, 24) active space in a biologically relevant system and show that small numbers of walkers are sufficient for a faithful sampling of the sc-NEVPT2 energy to chemical accuracy, despite the active space already exceeding the limits of practicality for traditional approaches. This raises prospects of an efficient stochastic solver for multireference chemical problems requiring large active spaces, with an accurate treatment of external orbitals.
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7 February 2020
Research Article|
February 03 2020
Efficient and stochastic multireference perturbation theory for large active spaces within a full configuration interaction quantum Monte Carlo framework
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JCP Editors' Choice 2020
Robert J. Anderson
;
Robert J. Anderson
1
Department of Physics, King’s College London
, Strand, London WC2R 2LS, United Kingdom
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Toru Shiozaki
;
Toru Shiozaki
2
Quantum Simulation Technologies, Inc.
, Cambridge, Massachusetts 02139, USA
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George H. Booth
George H. Booth
a)
1
Department of Physics, King’s College London
, Strand, London WC2R 2LS, United Kingdom
a)Author to whom correspondence should be addressed: [email protected]
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Robert J. Anderson
1
Toru Shiozaki
2
George H. Booth
1,a)
1
Department of Physics, King’s College London
, Strand, London WC2R 2LS, United Kingdom
2
Quantum Simulation Technologies, Inc.
, Cambridge, Massachusetts 02139, USA
a)Author to whom correspondence should be addressed: [email protected]
J. Chem. Phys. 152, 054101 (2020)
Article history
Received:
November 25 2019
Accepted:
January 09 2020
Citation
Robert J. Anderson, Toru Shiozaki, George H. Booth; Efficient and stochastic multireference perturbation theory for large active spaces within a full configuration interaction quantum Monte Carlo framework. J. Chem. Phys. 7 February 2020; 152 (5): 054101. https://doi.org/10.1063/1.5140086
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