Accelerating equilibrium isotope effect calculations. II. Stochastic implementation of direct estimators

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Quantum Chemistry

Background:

• Path integral calculations for equilibrium isotope effects are computationally intensive.
• Existing methods struggle with discretization, statistical, and thermodynamic integration errors.

Purpose of the Study:

• To reduce computational costs and improve accuracy in calculating isotope effects.
• To combine direct estimators with stochastic mass changes for enhanced performance.
• To compare different methods for calculating isotope effects on model systems and molecules.

Main Methods:

• Utilized a combination of direct estimators and stochastic mass changes.
• Applied path integral calculations to harmonic models, methane, and methanium.
• Evaluated isotope effects for CH4-xD_x/CH4 and CH5-xD_x+/CH5+ systems.

Main Results:

• The combined method successfully reduced statistical errors for larger isotope effects.
• Direct estimators showed effectiveness over a broader range of isotope effects than anticipated.
• A comprehensive comparison of different computational approaches was performed.

Conclusions:

• The hybrid approach offers a more efficient and accurate way to compute isotope effects.
• Understanding the behavior of direct estimators has implications for free energy calculations.
• This work advances the computational treatment of isotopic phenomena in chemistry.