Exact tunneling splittings from symmetrized path integrals

Area of Science:

• Quantum chemistry
• Computational physics
• Molecular dynamics

Background:

• Calculating ground-state tunneling splitting is crucial for understanding molecular behavior.
• Existing methods like instanton theory have limitations for certain molecular systems, such as those with low-barrier rotations.

Purpose of the Study:

• To develop a novel simulation technique for accurately calculating ground-state tunneling splitting patterns.
• To provide a method applicable to molecular systems where instanton theory is not valid.

Main Methods:

• A new simulation technique based on path-integral molecular dynamics.
• Rigorous projection of molecular systems onto their J = 0 rotational state using an "Eckart spring" within a ring polymer model.
• Thermodynamic integration at a single low temperature to obtain tunneling splitting from ratios of symmetrized partition functions.

Main Results:

• The developed method provides converged results formally identical to solving the full rovibrational Schrödinger equation.
• Simulations of hydronium and methanol using this technique show good agreement with experimental measurements and wavefunction-based calculations.
• The method is effective for studying low-barrier methyl rotations and other floppy modes.

Conclusions:

• The new path-integral molecular dynamics approach offers a robust way to calculate tunneling splitting.
• This method expands the capabilities for studying complex molecular dynamics, particularly for floppy modes.
• The technique is a valuable tool for computational chemistry and physics research.