Assessment of density functional approximations for the hemibonded structure of water dimer radical cation

TL;DR

This study evaluates various density functional approximations for accurately modeling the hemibonded structure of water dimer radical cation, highlighting the superior performance of long-range corrected double-hybrid functionals.

Contribution

It introduces three criteria for assessing functionals on hemibonded water cation and demonstrates the effectiveness of the omegaB97X-2(LP) functional in this context.

Findings

Long-range corrected double-hybrid functionals perform better.

OmegaB97X-2(LP) provides accurate binding and dissociation energies.

Conventional functionals often fail to dissociate the hemibonded structure correctly.

Abstract

Due to the severe self-interaction errors associated with some density functional approximations, conventional density functionals often fail to dissociate the hemibonded structure of water dimer radical cation (H2O)2+ into the correct fragments: H2O and H2O+. Consequently, the binding energy of the hemibonded structure (H2O)2+ is not well-defined. For a comprehensive comparison of different functionals for this system, we propose three criteria: (i) The binding energies, (ii) the relative energies between the conformers of the water dimer radical cation, and (iii) the dissociation curves predicted by different functionals. The long-range corrected (LC) double-hybrid functional, omegaB97X-2(LP) [J.-D. Chai and M. Head-Gordon, J. Chem. Phys., 2009, 131, 174105.], is shown to perform reasonably well based on these three criteria. Reasons that LC hybrid functionals generally work better than conventional density functionals for hemibonded systems are also explained in this work.