CC and EOM-CC provide the best answers for the largest number of accessible problems in quantum chemistry. However, there has been one application that has resisted accurate description. That is the problem of ‘dark states’, meaning states dominated by double excitations that have virtually zero transition moment with a closed shell ground state. The latter makes them ‘dark’ contrary to triplet states that are forbidden by spin symmetry. The latter triplets, along with singlet excited states dominated by single excitations are very accurate in EOM-CCSD, but for dark states, that level of treatment can have errors of several eV. It is well known, however, that once triple excitations are added into EOM-CCSDT, much better agreement is obtained. For some cases even quadruple excitations can also be important. However, these methods scaling as ~m8 and ~m10 are exceptionally demanding computationally. To assist routine spectral interpretation, one needs a method like EOM-CCSD that scales only as ~m6. This talk will present several such approaches toward this problem that maintain CCSD level calculations, including a new set of ‘addition by subtraction’ CC methods that well describe dark states.

Dark states in coupled-cluster (CC) theory and equation-of-motion CC

ACS Spring 2022

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