Dark Doubly Excited States with Modified Coupled Cluster Models: A Reliable Compromise between Cost and Accuracy?

Abstract

To treat doubly excited states, the treatment of triple excitations is considered necessary in the framework of equation-of-motion coupled cluster (EOM-CC) methods. We investigate models without explicit triples and seek quantitative measure for the performance of EOM based on CC with singles and doubles (CCSD) or modified CCSD (Distinguishable Cluster Approximation) approaches for states with predominant double excitation character. We also test the efficacy of including triples in perturbative manner through EOM-CCSD(T) and in an iterative way through EOM-CCSDT-3 method. Extended similarity transformed EOM-CCSD(EXT-STEOM-CCSD) method is also tested and provides superior quality results at comparatively low cost. We use the QUEST2 benchmark set of double excitations proposed by Loos et al. [J. Chem. Theory Comput.2019151939] to investigate the performance of methods such as EOM-CCSD, EOM-DCSD, EXT-STEOM-CCSD, ΔCCSD, and ΔDCSD. We also test a tailored CC approach, ΔpairCCD-TCCSD.

Aufbau Suppressed Coupled Cluster Theory for Electronically Excited States

“…Many widely used approaches, such as time-dependent density functional theory (TD-DFT), − equation-of-motion CC (EOM-CC), − and linear response CC (LR-CC), − rely on linear response and have only a limited ability to account for orbital relaxations or the new correlation effects created when an electron is transferred between different regions of a molecule. These issues are known to limit accuracy in charge transfer (CT), Rydberg, core, and double excitations. − State-specific approaches seek to improve matters by more fully tailoring their treatments to the needs of the excited state in question and have been developed for single determinant theories, − CI theory, − perturbation theory, , CC theory, ,− complete active space self-consistent field (CASSCF) theory, − and DFT. ,− In this context, ASCC offers a route to systematically improvable, state-specific CC treatments with good spin symmetry and working equations that closely mirror the ground state theory.…”

State-Specific Coupled-Cluster Methods for Excited States

“…126 This kind of approach has been also applied for low-order and cheaper CC methods such as pair coupledcluster doubles. 116,127,128 All of these works have been inspired by the discovery of multiple CC solutions. Z ̌ivkovićand Monkhorst were the first to study the conditions under which higher roots of the CC equations exist, 129,130 while Adamowicz and Bartlett explored the feasibility of reaching certain excited states of the LiH molecule.…”

Excited-State-Specific Pseudoprojected Coupled-Cluster Theory

“…Motivated in part by the limitations of linear response, there has been much recent work on excited-state-specific methods. Whether studying single-determinant wavefunctions, − configuration interaction (CI) wavefunctions, CC wavefunctions, − or DFT functionals, − the theme of locating higher energy, state-specific excited-state and open-shell roots is becoming increasingly prevalent across the field. Recognizing that, in the ground state, CCSD is a crucial stepping stone toward CCSD­(T) as well as a useful method in its own right, our focus here is to construct a CCSD-like excited-state-specific CC theory atop an excited-state mean field (ESMF) starting point − that, like HF in the ground state, has already taken care of state-specific orbital relaxations.…”