A domain-based local pair natural orbital implementation of the equation of motion coupled cluster method for electron attached states.

This work describes a domain-based local pair natural orbital (DLPNO) implementation of the equation of motion coupled cluster method for the computation of electron affinities (EAs) including single and double excitations. Similar to our earlier work on ionization potentials (IPs), the method reported in this study uses the ground state DLPNO framework and extends it to the electron attachment problem. While full linear scaling could not be achieved as in the IP case, leaving the Fock/Koopmans' contributions in the canonical basis and using a tighter threshold for singles PNOs allows us to compute accurate EAs and retain most of the efficiency of the DLPNO technique. Thus as in the IP case, the ground state truncation parameters are sufficient to control the accuracy of the computed EA values, although a new set of integrals for singles PNOs must be generated at the DLPNO integral transformation step. Using standard settings, our method reproduces the canonical results with a maximum absolute deviation of 49 meV for bound states of a test set of 24 molecules. Using the same settings, a calculation involving more than 4500 basis functions, including diffuse functions, takes four days on four cores, with only 48 min spent in the EA module itself.