Excitation Energies with Cost-Reduced Variant of the Active-Space EOMCCSDT Method: The EOMCCSDt-3̅ Approach

In this paper, we discuss the performance of several simplified variants of equation-of-motion coupled cluster method (EOMCC) with iterative inclusion of singles, doubles, and active-space triples (EOMCCSDt). In particular, we explore simplified EOMCCSDt approaches that enable one to generate the triply excited amplitudes in an on-the-fly manner. The original EOMCCSDt formulation has already demonstrated great success in encapsulating the most important excited-state correlation effects due to triples. In analogy to the original EOMCCSDT-3 formulation, the proposed approach can bypass the typical bottlenecks associated with the need for storing triply excited amplitudes. In this paper, we illustrate the performance of several approximate EOMCCSDt methods, named EOMCCSDt-3̅ and EOMCCSDt-3̅, on typical benchmark systems including C2, N2, ozone, ethene, and E-butadiene molecules. These new methods yield excitation energies close to the EOMCCSDt ones. The extrapolation of excitation energies for basis sets ranging from cc-pVDZ to cc-pV6Z for N2 and C2 shows very good convergence to the experimental results for states dominated by single excitations. The performance of the EOMCCSDt-3̅x approach is also compared with the results obtained with popular CCSDR(3) and CC3 approaches.