Adiabatic Switching Extended To Prepare Semiclassically Quantized Rotational-Vibrational Initial States for...

Area of Science:

• Chemical Physics
• Computational Chemistry
• Molecular Dynamics

Background:

• Accurate initial conditions are crucial for quasiclassical trajectory (QCT) calculations.
• Traditional methods like normal-mode sampling can suffer from rotational contamination and inaccuracies.
• Generating semiclassically quantized rovibrational states for polyatomic molecules remains a challenge.

Purpose of the Study:

• To present a novel approximation-free adiabatic switching method for generating semiclassically quantized rovibrational states.
• To provide improved initial conditions for QCT calculations of polyatomic molecules.
• To demonstrate the method's accuracy and efficiency using methane (CH4) as a model system.

Main Methods:

• Adiabatic switching is employed to prepare vibrational states by gradually introducing anharmonicity.
• An extension of the method is proposed to generate rovibrational states by slowly increasing angular momentum.
• The method avoids rotational contamination and approximation-free quantization.

Main Results:

• Ensembles generated for CH4 are insensitive to internal coordinate choices and are stationary.
• Mean energies of the ensembles closely match quantum mechanical values (zero-point energy and vibrational levels within 20 cm-1).
• Rotational levels (J=1-50) show excellent agreement (<1% error), with standard deviations consistently below 1%.

Conclusions:

• The adiabatic switching method provides classical state ensembles with superior properties compared to normal-mode sampling.
• This method offers a more appropriate and accurate approach for setting initial conditions in QCT simulations.
• The technique enhances the reliability and precision of molecular dynamics simulations for polyatomic systems.