Expectation Value-pCCD-Based Methods for Single-Electron Properties

Expectation-value-coupled cluster theory (XCC) offers
a simple
avenue for molecular property evaluation. However, its potential has
not been fully explored for the new computationally inexpensive CC
models, such as pair-coupled cluster doubles (pCCD) and post-pCCD
extensions. To that end, we implemented and explored one-electron
reduced density matrices in the explicitly connected commutator expansion
of the expectation value framework [J. Chem. Phys. 2006, 125, 184109] using pCCD, frozen pair Coupled
Cluster (fpCC), and frozen pair linearized Coupled Cluster (fpLCC)
variants. The expectation-value-based density matrices are calculated
directly using the cluster amplitudes and are computationally cheaper
than the corresponding response CC densities, as we bypass solving
the computationally expensive Λ-equations. The performance of
this approach, when combined with the pCCD-based methods, is assessed
against the dipole and quadrupole moments of molecules of a varying
chemical nature. We benchmarked our results against the response of
CCSD(T) using Hartree–Fock canonical orbitals and variationally
optimized pCCD orbitals. Our study highlights that localized pCCD
orbitals are a good choice for computing one-electron properties of
organic molecules.