Quantum Monte Carlo Calculations of Dihydrogen Binding Energetics on Ca Cations: an Assessment of Errors in Density Functionals for Weakly Bonded Systems

TL;DR

This study evaluates the accuracy of various electronic structure methods in predicting hydrogen binding energies on calcium ions, highlighting the importance of using advanced functionals like HFX+PBEC for reliable results.

Contribution

It provides a benchmark comparison between quantum Monte Carlo and density functional methods for weakly bonded hydrogen systems on calcium cations.

Findings

QMC shows no binding at short range for quadruple H2 molecules.

Common DFT functionals like LDA and B3LYP differ significantly from QMC results.

HFX+PBEC functional closely matches QMC binding energies.

Abstract

We investigate the binding of single and quadruple hydrogen molecules on a positively charged Ca ion. By comparing with benchmark quantum Monte Carlo (QMC) calculations we demonstrate wide variability in other more approximate electronic structure methods including common density functionals. Single determinant QMC calculations find no binding at short range by approximately 0.1 eV for the quadruple hydrogen molecule case, for a fixed hydrogen bond length of 0.77 Angstrom. Density functional calculations using common functionals such a LDA and B3LYP differ substantially from the QMC binding curve. We show that use of full Hartree-Fock exchange and PBE correlation(HFX+PBEC) obtains close agreement with the QMC results, both qualitatively and quantitatively. These results both motivate the use and development of improved functionals and indicate that caution is required applying electronic structure methods to weakly bound systems such as hydrogen storage materials based on metal ion decorated nanostructures.