Symmetry Breaking with the SCAN Density Functional Describes Strong Correlation in the Singlet Carbon Dimer

TL;DR

This paper demonstrates that the SCAN density functional, when combined with symmetry breaking, effectively describes strong correlation in the singlet carbon dimer, improving accuracy in atomization energy predictions.

Contribution

It shows that symmetry breaking with the SCAN functional can reliably capture strong correlation effects in molecules like C2, a novel approach for advanced density functionals.

Findings

SCAN accurately describes equilibrium bonds with strong correlation.

Symmetry breaking corrects under-binding in C2, aligning results with other molecules.

Symmetry-breaking insights enhance understanding of strong correlation phenomena.

Abstract

The SCAN (strongly constrained and appropriately normed) meta-generalized gradient approximation (meta-GGA), which satisfies all 17 exact constraints that a meta-GGA can satisfy, accurately describes equilibrium bonds that are normally correlated. With symmetry breaking, it also accurately describes some sd equilibrium bonds that are strongly correlated. While sp equilibrium bonds are nearly always normally correlated, the C2 singlet ground state is known to be a rare case of strong correlation in an sp equilibrium bond. Earlier work that calculated atomization energies of the molecular sequence B2, C2, O2, and F2 in the local spin density approximation (LSDA), the Perdew-Burke-Ernzerhof (PBE) GGA, and the SCAN meta-GGA, without symmetry breaking in the molecule, found that only SCAN was accurate enough to reveal an anomalous under-binding for C2. This work shows that spin symmetry breaking in singlet C2, the appearance of net up- and down-spin densities on opposite sides (not ends) of the bond, corrects that under-binding, with a small SCAN atomization-energy error more like that of the other three molecules, suggesting that symmetry-breaking with an advanced density functional might reliably describe strong correlation. This article also discusses some general aspects of symmetry breaking, and the insights into strong correlation that symmetry-breaking can bring.