  The efficiency of the recently proposed iCIPT2 [iterative configuration
interaction (iCI) with selection and second-order perturbation theory (PT2); J.
Chem. Theory Comput. 16, 2296 (2020)] for strongly correlated electrons is
further enhanced (by up to 20x) by using (1) a new ranking criterion for
configuration selection, (2) a new particle-hole algorithm for Hamiltonian
construction over randomly selected configuration state functions (CSF), and
(3) a new data structure for the quick sorting of the joint variational and
first-order interaction spaces. Meanwhile, the memory requirement is also
reduced greatly. As a result, this improved implementation of iCIPT2 can handle
one order of magnitude more CSFs than the previous version, as revealed by
taking the chromium dimer and an iron-sulfur cluster, [2Fe-2S], as examples.
