Toward Compact Selected Configuration Interaction Wave Functions with Quantum Monte Carlo─A Case Study of C₂

Abstract

The 1Σg + ground state of C2 is investigated using truncated CIPSI-Jastrow CSF wave functions with Hartree–Fock orbitals within the framework of variational and diffusion quantum Monte Carlo. The truncation is performed based on the absolute value of the CI coefficients, and the Jastrow, molecular orbitals, and CI parameters are either partially or fully reoptimized with respect to the variational energy. Excellent absolute as well as bond dissociation energies are obtained at DMC level with very compact, fully optimized wave functions. By studying the expansions in more detail, we observe a change in the CI picture when reoptimizing the antisymmetric part of the CIPSI-Jastrow wave functions. Furthermore, we demonstrate that a decrease in the VMC energy as well as an improvement of the nodal surface quality can be achievedwith the same expansion sizeif the CSFs are selected in the presence of a Jastrow correlation function, laying the foundation for a Jastrow selected CI scheme with quantum Monte Carlo.

Partial Separability of the Schrödinger Equation Combined with a Jastrow Factor

“…However, when larger systems are considered, the method becomes more complicated and less elegant. Nowadays, explicitly correlated wave functions are widely employed in electronic structure calculations of atoms and molecules by using numerical integration schemes such as those based on the Quantum Monte Carlo methods or other schemes such as the Explicitly Correlated R12/F12 or the Transcorrelated methods, see, e.g., refs − .…”