Oscillatory reaction cross sections caused by normal mode sampling in quasiclassical trajectory calculations

Area of Science:

• Physical Chemistry
• Chemical Dynamics
• Computational Chemistry

Background:

• The quasiclassical trajectory (QCT) method simulates atomic motion classically, incorporating quantum effects via semiclassical quantization of initial vibrational states.
• A key assumption is that the initial ensemble of classical molecular states should be stationary, mirroring its quantum counterpart.
• Normal mode approximation is the standard for sampling vibrational phase space in polyatomic molecules.

Purpose of the Study:

• To demonstrate that normal mode sampling generates a nonstationary ensemble for molecules like methane due to potential energy surface anharmonicity.
• To investigate the impact of this nonstationarity on QCT calculations for the CH4 + H reaction and its isotopologs.
• To propose and validate a method for correcting these artifacts.

Main Methods:

• Demonstration of nonstationary ensemble generation using normal mode sampling for methane.
• Analysis of reaction probabilities, cross sections, and excitation functions for the CH4 + H reaction.
• Development and application of a 'one-period averaging' method to correct for nonstationarity.

Main Results:

• Normal mode sampling yields a nonstationary ensemble, causing oscillations in reaction probabilities and erratic excitation functions for the CH4 + H reaction.
• The nonstationarity arises from the oscillating mean bond length of the breaking C-H bond.
• One-period averaging effectively removes these artifacts, producing physically reasonable reactivity parameters.

Conclusions:

• Standard normal mode sampling in QCT is flawed due to anharmonicity, leading to significant artifacts in reaction dynamics.
• The proposed one-period averaging method offers a straightforward solution to obtain accurate reaction probabilities and cross sections.
• This correction is crucial for reliable QCT studies employing normal mode sampling, particularly for reactions involving bond breaking.