Quadratic Unitary Coupled-Cluster Singles and Doubles Scheme: Efficient Implementation, Benchmark Study, and Formulation of an Extended Version

An
efficient implementation of the quadratic unitary coupled-cluster
singles and doubles (qUCCSD) scheme for calculations of electronic
ground and excited states using an unrestricted molecular spin–orbital
formulation and an efficient tensor contraction library is reported.
The accuracy of the qUCCSD scheme and the efficiency of the present
implementation are demonstrated using extensive benchmark calculations
of excitation energies and an application to S0 →
S1 vertical excitation energies for cis- and trans-4a,4b-dihydrotriphenylene. The qUCCSD scheme has been shown to provide
improved excitation energies compared with the UCC3 scheme formulated
based on perturbation theory. A UCC truncation scheme that can provide
excitation energies correct through the fourth order is also presented
to further improve the accuracy of the qUCCSD scheme.