Using experimental data and a contracted basis Lanczos method to determine an accurate methane potential energy...

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Quantum Mechanics

Background:

• Accurate potential energy surfaces (PES) are crucial for understanding molecular vibrations and spectra.
• Previous ab initio calculations for methane (CH4) provided a starting point but required refinement for high accuracy.
• Experimental vibrational data offers benchmarks for validating theoretical models.

Purpose of the Study:

• To develop a highly accurate potential energy surface (PES) for methane (CH4).
• To improve the reproduction of experimentally determined vibrational energy levels.
• To aid in the spectral analysis of methane, particularly for higher energy polyads like the Tetradecad.

Main Methods:

• Started with the ab initio potential energy surface (PES) of Schwenke and Partridge (2001).
• Adjusted five parameters of the initial PES.
• Refined the PES to reproduce 40 experimentally determined vibrational levels of CH4, including levels up to the Octad and in the Tetradecad.

Main Results:

• Reduced the root mean square deviation (RMSD) for 40 vibrational levels from 4.80 cm⁻¹ to 0.28 cm⁻¹.
• The new PES accurately predicts vibrational levels for isotopologues CH3D, CHD3, and (13)CH4, with errors generally within 1 cm⁻¹.
• Demonstrated the utility of the new PES for analyzing complex spectral regions like the Tetradecad.

Conclusions:

• The refined potential energy surface (PES) provides a significant improvement in accuracy for methane.
• The new PES is a valuable tool for high-resolution spectroscopy and theoretical studies of methane.
• The methodology can be applied to refine PES for other molecules.