Geometry optimization using tuned and balanced redistributed charge schemes for combined quantum mechanical and...

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Molecular Mechanics

Background:

• Combined quantum mechanical and molecular mechanical (QM/MM) methods are essential for simulating large molecular systems.
• Accurate treatment of the QM/MM boundary is critical for reliable results.
• Existing charge redistribution algorithms have limitations in representing electrostatic interactions at the QM/MM interface.

Purpose of the Study:

• To develop and evaluate new algorithms for treating the QM/MM boundary.
• To improve the accuracy of QM/MM calculations, particularly for systems with polar bonds.
• To enhance the realistic representation of electrostatic interactions across the QM/MM interface.

Main Methods:

• Geometry optimizations using tuned and balanced redistributed charge algorithms.
• Implementation of the tuned and balanced redistributed charge-2 (TBRC2) scheme.
• Development and implementation of the tuned and balanced smeared redistributed charge (TBSRC) scheme, incorporating charge smearing.
• Testing schemes on various bond types (C-C, C-N, C-O, etc.) at the QM/MM boundary.

Main Results:

• Both TBRC2 and TBSRC schemes show excellent agreement with full QM calculations.
• Mean unsigned error (MUE) for deprotonation energy is 1.6 kcal/mol for both schemes.
• MUE for optimized bond lengths near the boundary is 0.015 Å for TBRC2 and 0.021 Å for TBSRC.
• Charge smearing in TBSRC is beneficial when redistributed charges are close to the QM region.

Conclusions:

• The TBRC2 and TBSRC schemes provide accurate and reliable QM/MM boundary treatments.
• These new schemes are particularly important for QM/MM boundaries passing through polar bonds.
• The developed methods significantly improve the accuracy of QM/MM simulations, enabling more precise molecular modeling.