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.
