Physics

Electron

Mathematics

Algorithm

Mathematical optimization

Hamiltonian (control theory)

Coupled cluster

Perturbation theory (quantum mechanics)

Configuration interaction

Molecule

Computer science

Quantum mechanics

Journal of Chemical Theory and Computation

The efficiency of the recently proposed iCIPT2 [iterative configuration interaction (iCI) with selection and second-order perturbation theory (PT2); J. Chem. Theory Comput. 2020, 16, 2296] for strongly correlated electrons is further enhanced (by up to 20×) 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 variational and first-order interaction spaces. Meanwhile, the memory requirement is also significantly reduced. As a result, this improved implementation of iCIPT2 can handle 1 order of magnitude more CSFs than the previous version, as revealed by taking the chromium dimer and an iron–sulfur cluster, [Fe2S2(SCH3)]42–, as examples.

Further Development of iCIPT2 for Strongly Correlated Electrons