Adiabatic Trajectory Approximation within the Framework of Mixed Quantum/Classical Theory.

Area of Science:

• Computational chemistry
• Chemical physics
• Molecular dynamics

Background:

• Inelastic molecular collisions are computationally intensive.
• Accurate simulation requires solving complex quantum and classical equations of motion.
• Mixed Quantum/Classical Theory (MQCT) offers a hybrid approach.

Purpose of the Study:

• To develop and validate a hierarchy of approximate methods within MQCT for inelastic molecular collisions.
• To investigate a specific decoupled approach: adiabatic trajectory MQCT (AT-MQCT).
• To assess the computational efficiency and accuracy of the AT-MQCT method.

Main Methods:

• Proposed a hierarchy of approximate methods for solving MQCT equations of motion.
• Developed a decoupled approach where translational and internal molecular motions are treated separately.
• Utilized an adiabatic trajectory approximation for pre-computing trajectories.
• Employed an expanded basis set and efficient step-size adjustment for quantum transition probability calculations.

Main Results:

• The AT-MQCT method achieved a 2-order of magnitude (100x) speedup in computations for H2O + H2 rotationally inelastic scattering.
• Benchmarking confirmed the accuracy of the AT-MQCT results against coupled quantum/classical and full-quantum calculations.
• The approximate propagation scheme demonstrated high fidelity compared to more rigorous methods.

Conclusions:

• The AT-MQCT method is a computationally efficient and accurate tool for simulating inelastic molecular collisions.
• This approach offers a promising avenue for the study of molecular energy exchange.
• The developed hierarchy of methods provides flexibility for various computational chemistry applications.