Noniterative triple excitations contributions in equation-of-motion coupled-cluster theory for singlet and triplet excitation energies of closed-shell systems

In this work, we report the implementation of the equation-of-motion coupled-cluster method with noniterative triple excitations contributions [EOM-CCSD(T)(a)*] for spin-singlet and spin-triplet excited states based on a closed-shell reference. We also propose the l-EOM-CCSD(T)(a)* method, where {R1, R2} is approximated by {L1, L2} in EOM-CCSD(T)(a)*. In the implementation, spin and spatial symmetries are exploited to reduce computational cost. Vertical and adiabatic excitation energies, numerical energy gradients, optimized structures, and harmonic frequencies for some low-lying states of a series of organic molecules have been calculated using different EOM-CC methods. Results show that EOM-CCSD(T)(a)* and l-EOM-CCSD(T)(a)* are able to provide a highly accurate description of excited states, and differences between their singlet excitation energies and results of CCSDR(3) are very small. l-EOM-CCSD(T)(a)* has a lower computational cost and is recommended for calculating excited-state properties of organic molecules. In addition, using the CCSD(T) energy as the ground-state energy in EOM-CCSD(T)(a)* can improve excited-state properties.