Flexible boundary layer using exchange for embedding theories. I. Theory and implementation.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Physical Chemistry

Background:

• Embedding theory, particularly Quantum Mechanical/Molecular Mechanics (QM/MM), is vital for studying complex systems.
• A key challenge is maintaining QM/MM separation in systems with diffusible particles, such as solvents, which can lead to inaccurate results.

Purpose of the Study:

• To develop a novel method that addresses the challenge of particle inter-diffusion in QM/MM simulations.
• To enable the accurate study of systems requiring QM treatment of diffusible particles, like solvent-supported electronic states or proton transfer reactions.

Main Methods:

• Introduction of a new method: Flexible Boundary Layer using Exchange (FlexiBLE).
• FlexiBLE employs a biasing potential to maintain QM/MM separation, leveraging particle exchange invariance to preserve ensemble averages.
• Utilizes a tree algorithm for efficient handling of QM and MM particle exchanges, ensuring computational efficiency.

Main Results:

• FlexiBLE effectively prevents the loss of QM/MM separation caused by particle inter-diffusion.
• The method demonstrates computational cost similar to Molecular Mechanics (MM) force fields, adding negligible overhead.
• Molecular dynamics simulations with FlexiBLE conserve total energy, and dynamics within the QM region remain unaffected by the bias.

Conclusions:

• FlexiBLE successfully overcomes a significant limitation in embedding theory for systems with diffusible particles.
• The method broadens the applicability of QM/MM simulations to a wider range of chemical phenomena.
• FlexiBLE offers a computationally efficient and accurate approach for complex chemical system simulations.