PNNL

Abstract

In this paper we discuss the performance of the 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 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, and the ozone 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.