Electrostatic considerations affecting the calculated HOMO-LUMO gap in protein molecules

TL;DR

This paper investigates the causes of vanishing HOMO-LUMO gaps in protein and water cluster systems, revealing that electrostatic artifacts, not functional limitations, are responsible, and proposes solutions to maintain the gap.

Contribution

It demonstrates that electrostatic artifacts cause vanishing HOMO-LUMO gaps in large biomolecular systems and offers practical methods to correct this issue.

Findings

Electrostatic artifacts lead to vanishing HOMO-LUMO gaps.

Solutions are provided to preserve the gap in large systems.

Implications for biomolecular electronic property simulations.

Abstract

A detailed study of energy differences between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gaps) in protein systems and water clusters is presented. Recent work questioning the applicability of Kohn-Sham density-functional theory to proteins and large water clusters (E. Rudberg, J. Phys.: Condens. Mat. 2012, 24, 072202) has demonstrated vanishing HOMO-LUMO gaps for these systems, which is generally attributed to the treatment of exchange in the functional used. The present work shows that the vanishing gap is, in fact, an electrostatic artefact of the method used to prepare the system. Practical solutions for ensuring the gap is maintained when the system size is increased are demonstrated. This work has important implications for the use of large-scale density-functional theory in biomolecular systems, particularly in the simulation of photoemission, optical absorption and electronic transport, all of which depend critically on differences between energies of molecular orbitals.