New and Efficient Implementation of CC3

We present a new and efficient implementation of the closed shell coupled cluster singles and doubles with perturbative triples method (CC3) in the electronic structure program eT. Asymptotically, a ground state calculation has an iterative cost of 4nV4nO3 floating point operations (FLOP), where nV and nO are the number of virtual and occupied orbitals, respectively. The Jacobian and transpose Jacobian transformations, required to iteratively solve for excitation energies and transition moments, both require 8nV4nO3 FLOP. We have also implemented equation of motion (EOM) transition moments for CC3. The EOM transition densities require recalculation of triples amplitudes, as nV3nO3 tensors are not stored in memory. This results in a noniterative computational cost of 10nV4nO3 FLOP for the ground state density and 26nV4nO3 FLOP per state for the transition densities. The code is compared to the CC3 implementations in CFOUR, DALTON, and PSI4. We demonstrate the capabilities of our implementation by calculating valence and core excited states of l-proline.