Adaptive-Partitioning Multilayer Dynamics Simulations: 2. Implementations of the Permuted and Interpolated...

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Molecular Mechanics

Background:

• Adaptive-partitioning multilayer QM/MM methods offer flexible computational approaches by integrating different levels of theory (Q1, Q2, molecular mechanics/MM).
• These methods, similar to ONIOM, dynamically reclassify atoms during simulations, requiring smooth transitions between layers using buffer zones and energy interpolation.
• Two formalisms, permuted adaptive-partitioning (PAP) and interpolated adaptive-partitioning (IAP), exist for calculating Q1/Q2 interactions, with IAP utilizing alchemical quantum calculations.

Purpose of the Study:

• To thoroughly investigate and identify the causes of energy irregularities observed in the IAP formalism near the buffer/Q2 boundary.
• To derive and implement analytical gradients for both PAP and IAP formalisms to assess their performance.
• To analyze the behavior of energy and gradient discontinuities in IAP and propose strategies for their mitigation.

Main Methods:

• Analysis of IAP energy calculations, focusing on irregularities near the buffer/Q2 interface.
• Implementation and testing of a cutoff for weighted atomic orbital coefficients to address basis-set linear dependencies.
• Derivation and numerical implementation of energy gradients for both PAP and IAP methods.
• Computational testing on water cluster models to evaluate energy and gradient smoothness.

Main Results:

• Irregularities in IAP energy were identified and attributed to basis-set linear dependencies, effectively suppressed by a cutoff for atomic orbital coefficients.
• PAP gradients were found to be perfectly smooth across layers.
• Minor discontinuities were observed in IAP gradients at the buffer/Q2 boundary, which decreased with increased buffer size and larger basis sets.

Conclusions:

• Basis-set linear dependencies are a key factor causing IAP energy irregularities, manageable with appropriate cutoffs.
• While PAP offers smooth gradients, IAP exhibits controllable discontinuities that can be minimized by adjusting the buffer zone and basis set size.
• The findings provide insights into refining multilayer QM/MM methods for more accurate and stable simulations.