Impact of high-rank excitations on accuracy of the unitary coupled cluster downfolding formalism

TL;DR

This paper assesses how high-rank excitations influence the accuracy of the Hermitian downfolding method using the DUCC approach on benchmark systems, highlighting the importance of higher-rank amplitudes in strongly correlated regimes.

Contribution

It demonstrates that incorporating higher-rank external excitations improves the accuracy of the DUCC downfolding formalism in strongly correlated systems.

Findings

Higher-rank SR-CC external amplitudes are necessary for accurate energies in strongly correlated regimes.

The approach can mitigate issues related to the non-variational nature of standard SR-CC methods.

Exact operator representations are achieved using occupation-number-representation codes.

Abstract

In this paper, we evaluate the accuracy of the Hermitian form of the downfolding procedure utilizing the double unitary coupled cluster Ansatz (DUCC) on the H6 and H8 benchmark systems. The computational infrastructure employs the occupation-number-representation codes to construct the matrix representation of arbitrary second-quantized operators, enabling the exact representation of exponentials of various operators. The tests utilize external excitations estimated from standard single-reference coupled cluster methods (SR-CC) to demonstrate that higher-rank SR-CC external amplitudes were necessary to describe the energies in the strongly correlated regime adequately. We show that this approach can offset problems of the corresponding SR-CC theories associated with losing the variational character of corresponding energies.