Abstract
An algorithm to perform stochastic generalized active space
calculations, Stochastic-GAS, is presented, that
uses the Slater determinant based FCIQMC algorithm as configuration
interaction eigensolver.
Stochastic-GAS allows the construction and stochastic optimization
of preselected truncated configuration interaction wave functions,
either to reduce the computational costs of large active space wave
function optimizations, or to probe the role of specific electron
correlation pathways.
As for the conventional GAS procedure, the preselection of the
truncated wave function is based on the selection of multiple
active subspaces while imposing restrictions on the interspace excitations.
Both local and cumulative minimum and maximum occupation number
constraints are supported by Stochastic-GAS.
The occupation number constraints are efficiently encoded in precomputed
probability distributions, using the precomputed heat bath algorithm,
which removes nearly all runtime overheads of GAS.
This strategy effectively allows the FCIQMC dynamics to a priori exclude
electronic configurations that are not allowed by GAS restrictions.
Stochastic-GAS reduced density matrices are stochastically sampled,
allowing orbital relaxations via Stochastic-GASSCF, and direct evaluation
of properties that can be extracted from density matrices, such as the
spin expectation value.
Three test case applications have been chosen to demonstrate the
flexibility of Stochastic-GAS:
(a) the Stochastic-GASSCF optimization of a
stack of five benzene molecules, that shows the applicability of
Stochastic-GAS towards fragment-based chemical systems;
(b) an uncontracted stochastic MRCISD calculation
that correlates 96 electrons and 159 molecular orbitals, and uses a large
(32, 34) active space reference wave function for an Fe(II)-porphyrin
model system, showing how GAS can be applied to systematically
recover dynamic electron correlation, and how in the specific case
of the Fe(II)-porphyrin dynamic correlation further differentially stabilizes
the triplet over the quintet spin state;
(c) the study of an Fe4S4 cluster's spin-ladder energetics
via highly truncated stochastic-GAS wave functions, where we show
how GAS can be applied to understand the competing
spin-exchange and charge-transfer correlating mechanisms in
stabilizing different spin-states.
Supplementary materials
Title
Supporting Information
Description
The Supporting_Information.pdf describes computational details and the other available files that are necessary to reproduce our results.
In addition one finds tables of all results and a derivation of the working equation for the evaluation of the $\hat{S}^2$ expectation value.
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Title
Additional files
Description
This tar-archive contains coordinates, inputs, and starting orbitals that are necessary to reproduce our results.
They are described in the Supporting Information.
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