Benchmarking theoretical electronic structure methods with photoemission orbital tomography

TL;DR

This study combines experimental photoemission orbital tomography with density functional theory calculations to benchmark the accuracy of various exchange-correlation functionals in predicting orbital energies at an organic/metal interface.

Contribution

It provides a detailed orbital-by-orbital comparison between experiment and theory, benchmarking functional performance for orbital energy alignment.

Findings

Identified 25 orbitals of bisanthene on Cu(110) with measured energies.

Benchmarking shows varying accuracy of functionals in predicting orbital energies.

Provides insights into the suitability of different DFT functionals for interface studies.

Abstract

In the past decade, photoemission orbital tomography (POT) has evolved into a powerful tool to investigate the electronic structure of organic molecules adsorbed on (metallic) surfaces. By measuring the angular distribution of photoelectrons as a function of binding energy and making use of the momentum-space signature of molecular orbitals, POT leads to an orbital-resolved picture of the electronic density of states at the organic/metal interface. In this combined experimental and theoretical work, we apply POT to the prototypical organic $\pi$-conjugated molecule bisanthene (C$_{28}$H$_{14}$) which forms a highly oriented monolayer on a Cu(110) surface. Experimentally, we identify an unprecedented number of 13 $\pi$ and 12 $\sigma$ orbitals of bisanthene and measure their respective binding energies and spectral lineshapes at the bisanthene/Cu(110) interface. Theoretically, we perform density functional calculations for this interface employing four widely used exchange-correlation functionals from the families of the generalized gradient approximations as well as global and range-separated hybrid functionals. By analyzing the electronic structure in terms of orbital-projected density of states, we arrive at a detailed orbital-by-orbital assessment of theory vs. experiment. This allows us to benchmark the performance of the investigated functionals with regards to their capability of accounting for the orbital energy alignment at organic/metal interfaces.