Calculation of Energy Level Alignment and Interface Electronic Structure in Molecular Junctions beyond DFT

• Published in: Journal of physical chemistry. C Vol. 125; no. 46; pp. 25825 - 25831
• Main Authors: Montes, Enrique, Vázquez, Héctor
• Format: Journal Article
• Language: English
• Published: American Chemical Society 25.11.2021
• Subjects: C: Physical Properties of Materials and Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.1c07407

In atomistic simulations of molecular junctions, it is important to develop methods beyond density-functional theory (DFT) to describe the interface electronic structure and alignment of frontier molecular orbitals accurately. Here we describe a first-principles approach for molecular junctions that extends the DFT+Σ method, an approximate scheme based on self-energy corrections. The DFT+Σtot method presented here acts on junction states and introduces corrections to DFT-based molecular frontier orbitals not only on the molecular subspace but on the whole junction Hamiltonian. These self-energy corrections are scaled according to the molecular character of each junction wave function, a character which is given by projection coefficients between molecular orbitals and junction states. We illustrate this formalism in three paradigmatic weakly interacting single molecule junctions. Despite the weak metal/molecule interaction, calculated projection coefficients show metal/molecule hybridization resulting in molecular orbital features that can be significantly broadened over a wide energy range. Scaling of self-energy corrections to molecular levels seamlessly accounts for this hybridization and spectral delocalization. The resulting DFT+Σtot electronic structure brings molecular frontier orbital energies to values comparable to the G0W0 and WKM approaches. By considering the whole spectral distribution of molecular orbitals, DFT+Σtot shifts not just the main molecular peaks but modifies the whole junction electronic structure.