Wannier-function-based constrained DFT with nonorthogonality-correcting Pulay forces in application to the reorganization effects in graphene-adsorbed pentacene

TL;DR

This paper derives and implements exact Pulay force corrections for nonorthogonal localized orbitals in constrained DFT, enabling accurate calculations of reorganization energies for molecules on surfaces, exemplified by pentacene on graphene.

Contribution

It introduces a rigorous correction for Pulay forces in nonorthogonal basis sets within constrained DFT and demonstrates its application to surface-adsorbed molecules.

Findings

Corrected Pulay forces improve force accuracy in nonorthogonal basis DFT.

Reorganization energy of pentacene on graphene is lower than in gas phase.

Tensorially invariant population analysis yields near-integer populations for adsorbates.

Abstract

Pulay terms arise in the Hellman-Feynman forces in electronic structure calculations when one employs a basis set made of localized orbitals that move with their host atoms. If the total energy of the system depends on a subspace population defined in terms of the localized orbitals across multiple atoms, then unconventional Pulay terms will emerge due to the variation of the orbital nonorthogonality with ionic translation. Here, we derive the required exact expressions for such terms, which cannot be eliminated by orbital orthonormalization. We have implemented these corrected ionic forces within the linear-scaling density functional theory (DFT) package ONETEP, and have used constrained DFT to calculate the reorganization energy of a pentacene molecule adsorbed on a graphene flake. The calculations are performed by including ensemble DFT, corrections for periodic boundary conditions, and empirical Van der Waals interactions. For this system we find that tensorially invariant population analysis yields an adsorbate subspace population that is very close to integer-valued when based upon nonorthogonal Wannier functions, and also but less precisely when using pseudoatomic functions. Thus, orbitals can provide a very effective population analysis for constrained DFT. Our calculations show that the reorganization energy of the adsorbed pentacene is typically lower than that of pentacene in the gas phase. We attribute this effect to steric hindrance.