New and Efficient Implementation of CC3

Alexander C Paul; Rolf H Myhre; Henrik Koch

doi:10.1021/acs.jctc.0c00686

New and Efficient Implementation of CC3

J Chem Theory Comput. 2021 Jan 12;17(1):117-126. doi: 10.1021/acs.jctc.0c00686. Epub 2020 Dec 2.

Authors

Alexander C Paul¹, Rolf H Myhre¹, Henrik Koch^{1

2}

Affiliations

¹ Department of Chemistry, Norwegian University of Science and Technology, NTNU, 7491 Trondheim, Norway.
² Scuola Normale Superiore, Piazza dei Cavaleri 7, 56126 Pisa, Italy.

Abstract

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 e^T. Asymptotically, a ground state calculation has an iterative cost of 4n_V⁴n_O³ floating point operations (FLOP), where n_V and n_O 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 8n_V⁴n_O³ FLOP. We have also implemented equation of motion (EOM) transition moments for CC3. The EOM transition densities require recalculation of triples amplitudes, as n_V³n_O³ tensors are not stored in memory. This results in a noniterative computational cost of 10n_V⁴n_O³ FLOP for the ground state density and 26n_V⁴n_O³ 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.