Exact two-component equation-of-motion coupled-cluster singles and doubles method using atomic mean-field spin-orbit integrals.

A new semi-atomic-orbital- based algorithm for a two-component spin-orbit (SO) equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method using mean-field SO integrals is reported. The new algorithm removes the major computational bottlenecks of a SO-EOM-CCSD calculation associated with the evaluation, storage, and processing of the H¯ab,cd elements in the similarity-transformed Hamiltonian involving four virtual orbital labels. The partial recovery of spin symmetry in the present algorithm reduces the storage requirement by an order of magnitude and the floating point operation count for the evaluation of the ladder-like term by a factor of three to four. EOM-CCSD calculations of excited states in the triiodide ion (I3 -) using the exact two-component Hamiltonian in combination with atomic mean-field SO integrals (X2CAMF) are reported as a validation of the implementation and also as a demonstration of the capability of the new algorithm to correlate extended virtual spaces. X2CAMF-EOM-CCSD calculations of the ground and excited states in As2, Sb2, and Bi2 are also presented and compared with the available experimental studies. An analysis based on the computed spectroscopic constants as well as the compositions of the excited-state wavefunctions strongly supports a new assignment for the lowest 2u and 0u - levels in the photoelectron spectrum of Bi2.