The Role of High-Order Electron Correlation Effects in a Model System for Non-valence Correlation-bound Anions

TL;DR

This study compares advanced quantum Monte Carlo and coupled cluster methods to accurately calculate electron binding energies in a model water cluster, emphasizing the importance of high-order electron correlation effects in non-valence anion states.

Contribution

It demonstrates the significance of high-order correlation effects and basis set choices in accurately modeling non-valence correlation-bound anions using DMC, AFQMC, and EOM-CC methods.

Findings

DMC can recover from HF trial wave functions that collapse onto continuum solutions.

Including triples and diffuse functions improves EBE accuracy for non-valence anions.

DMC and EOM-CC yield consistent EBE estimates when using suitable trial wave functions.

Abstract

The diffusion Monte Carlo (DMC), auxiliary field quantum Monte Carlo (AFQMC), and equation-of-motion coupled cluster (EOM-CC) methods are used to calculate the electron binding energy (EBE) of the non-valence anion state of a model (H$_2$O)$_4$ cluster. Two geometries are considered, one at which the anion is unbound and the other at which it is bound in the Hartree-Fock (HF) approximation. It is demonstrated that DMC calculations can recover from the use of a HF trial wave function that has collapsed onto a discretized continuum solution, although larger electron binding energies are obtained when using a trial wave function for the anion that provides a more realistic description of the charge distribution, and, hence, of the nodal surface. For the geometry at which the cluster has a non-valence correlation-bound anion, both the inclusion of triples in the EOM-CC method and the inclusion of supplemental diffuse d functions in the basis set are important. DMC calculations with suitable trial wave functions give EBE values in good agreement with our best estimate EOM-CC result. AFQMC using a trial wave function for the anion with a realistic electron density gives a value of the EBE nearly identical to the EOM-CC result when using the same basis set. For the geometry at which the anion is bound in the HF approximation, the inclusion of triple excitations in the EOM-CC calculations is much less important. The best estimate EOM-CC EBE value is in good agreement with the results of DMC calculations with appropriate trial wave functions.