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 which enable one to generate the triply excited amplitudes in on-the-fly manner. The original EOMCCSDt formulation has already demonstrated a 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 by-pass 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-3 and EOMCCSDt-3x, 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-3x approach is also compared with the results obtained with popular CCSDR(3) and CC3 approaches.

Excitation energies with cost-reduced variant of the active-space EOMCCSDT method: The EOMCCSDt-3 approach

248th National Meeting (2014)

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