The interplay between strong correlation and adsorption distances

TL;DR

This paper investigates how strong electron correlation affects adsorption distances of transition metal atoms on surfaces, revealing that correlation effects can significantly shift predicted equilibrium positions.

Contribution

It demonstrates that combining DFT with the Anderson impurity model or DFT+U provides more accurate adsorption distances by accounting for correlation effects often missed by standard DFT.

Findings

Correlation energy increases with larger adsorption distances.

DFT+AIM and DFT+U predict larger adsorption distances than DFT.

Correlation effects are linked to specific Co 3d orbitals.

Abstract

Adsorbed transition metal atoms can have partially filled $d$- or $f$-shells due to strong on-site Coulomb interaction. Capturing all effects originating from electron correlation in such strongly correlated systems is a challenge for electronic structure methods. It requires a sufficiently accurate description of the atomistic structure (in particular bond distances and angles), which is usually obtained from Kohn-Sham density functional theory (DFT), which due to the approximate nature of the exchange-correlation functional may provide an unreliable description of strongly correlated systems. To elucidate the consequences of this popular procedure, we apply a combination of DFT with the Anderson impurity model (AIM), as well as DFT+U for a calculation of the potential energy surface along the Co/Cu(001) adsorption coordinate, and compare the results with those obtained from DFT. The adsorption minimum is shifted towards larger distances by applying DFT+AIM, or the much cheaper DFT+U method, compared to the corresponding spin-polarized DFT results, by a magnitude comparable to variations between different approximate exchange-correlation functionals (0.08 to 0.12 Angstrom). This shift originates from an increasing correlation energy at larger adsorption distances, which can be traced back to the Co 3$d_{xy}$ and 3$d_{z2}$ orbitals being more correlated as the adsorption distance is increased.