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

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Biomolecular Simulations

Background:

• Recent studies have questioned the accuracy of Kohn-Sham density-functional theory (KS-DFT) for calculating highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gaps in large systems like proteins and water clusters.
• These previous works often attributed vanishing HOMO-LUMO gaps to the specific exchange functional used within KS-DFT.
• Accurate HOMO-LUMO gaps are crucial for understanding electronic properties and simulating phenomena such as photoemission and optical absorption.

Purpose of the Study:

• To investigate the cause of vanishing HOMO-LUMO gaps observed in large protein and water cluster systems using KS-DFT.
• To identify and demonstrate practical solutions for maintaining accurate HOMO-LUMO gaps as system size increases.
• To clarify the implications of these findings for the application of large-scale KS-DFT in biomolecular simulations.

Main Methods:

• Detailed analysis of energy differences between highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gaps).
• Investigation of system preparation methods to identify potential artifacts.
• Demonstration of practical solutions to ensure gap stability with increasing system size.

Main Results:

• The study demonstrates that vanishing HOMO-LUMO gaps in large protein and water cluster systems are primarily an electrostatic artifact.
• This artifact arises from the specific method used for system preparation, rather than the treatment of exchange in the density functional.
• Practical strategies were successfully demonstrated to maintain accurate HOMO-LUMO gaps even for large-scale systems.

Conclusions:

• The observed vanishing HOMO-LUMO gaps in large biomolecular systems using KS-DFT are an artifact of system preparation, not the exchange functional.
• This finding corrects a common misconception and provides a pathway for more reliable electronic structure calculations in biomolecules.
• The work has significant implications for accurately simulating photoemission, optical absorption, and electronic transport in biological systems.