It is well established that the exponential wave function ansatz of coupled-cluster (CC) theory and its extensions to excited, open-shell, and multireference states are among the most efficient ways of incorporating many-electron correlation effects in molecular applications. In this talk, we will discuss novel ways of obtaining accurate energetics equivalent to high-level CC calculations, such as CCSDT, CCSDTQ, and EOMCCSDT, at the small fractions of the computational costs, even when multireference correlation effects become significant, which result from the merger of moment expansions defining the CC(<i>P</i>;<i>Q</i>) formalism with Quantum Monte Carlo (QMC) propagations in the many-electron Hilbert space and with the selected configuration interaction (CI) approach abbreviated as CIPSI. We will also demonstrate that one can use the merger of the CC and stochastic and selected CI ideas to extract the exact, full CI (FCI), or near-exact energetics out of the early stages of FCIQMC propagations or from the initial iterations of CIPSI with the help of the relatively inexpensive polynomial steps similar to CCSD, eliminating exponential complexity of conventional FCI Hamiltonian diagonalizations altogether.

Approaching exact quantum chemistry by semi-stochastic and selected-CI-driven coupled-cluster computations

ACS Spring 2022

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