A density functional theory for symmetric radical cations from bonding to dissociation

TL;DR

This paper introduces a new class of long-range corrected density functionals that accurately describe symmetric radical cations, overcoming previous failures of approximate density functional theories in dissociation scenarios.

Contribution

The authors develop system-dependent long-range corrected density functionals that prevent symmetry breaking and improve the description of radical cation dissociation.

Findings

Enhanced accuracy in bond property predictions for R2+ cations

Better modeling of asymptotic potential curves and atomic polarizability

Comparison with experimental data shows significant improvements

Abstract

It is known for quite some time that approximate density functional (ADF) theories fail disastrously when describing the dis-sociative symmetric radical cations R2+. Considering this dissociation limit, previous work has shown that Hartree-Fock (HF) theory favors the R+1--R0 charge distribution while DF approximations favor the R+0.5 -- R+0.5. Yet, general quantum mechanical principles indicate that both these (as well as all intermediate) average charge distributions are asymptotically energy degenerate. Thus HF and ADF theories mistakenly break the symmetry but in a contradicting way. In this letter we show how to construct system-dependent long-range corrected (LC) density functionals that can successfully treat this class of molecules, avoiding the spurious symmetry breaking. Examples and comparisons to experimental data is given for R=H, He and Ne and it is shown that the new LC theory improves considerably the theoretical description of the R2+ bond properties, the long range form of the asymptotic potential curve as well as the atomic polarizability. The broader impact of this finding is discussed as well and it is argued that the widespread semi-empirical approach which advocates treating the LC parameter as a system-independent parameter is in fact inappropriate under general circumstances.