Daniel Cole

Electrostatic Considerations Affecting the Calculated HOMO-LUMO Gap in Protein Molecules

Greg Lever,1 Daniel J. Cole,1,2 Nicholas D. M. Hine,3 Peter D. Haynes,3 Mike C. Payne1
1Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK
2Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States
3Departments of Materials and Physics, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom

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.

Journal of Physics: Condensed Matter (Fast Track Communications) 25, 152101 (2013)