Fast vibrational configuration interaction using generalized curvilinear coordinates and self-consistent basis.

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Quantum Mechanics

Background:

• Accurate calculation of vibrational anharmonic frequencies is crucial for understanding molecular dynamics.
• Existing methods struggle with molecular systems exhibiting large-amplitude motions, such as methyl-group torsion.
• Efficient evaluation of ab initio potential-energy surfaces (PES) is computationally demanding.

Purpose of the Study:

• To develop a general formalism for computing vibrational bound states in molecular systems with large-amplitude motion.
• To introduce a numerical kinetic-energy operator approach coupled with direct vibrational self-consistent field (VSCF)/vibrational configuration interaction (VCI).
• To validate and apply the new method to methanol, focusing on torsional frequencies.

Main Methods:

• Coupling a numerical kinetic-energy operator approach with direct vibrational self-consistent field (VSCF)/vibrational configuration interaction (VCI).
• Utilizing fast-VSCF for efficient direct evaluation of the ab initio potential-energy surface (PES).
• Employing curvilinear coordinates for describing large-amplitude molecular motions.

Main Results:

• The developed formalism accurately computes vibrational anharmonic frequencies for systems with large-amplitude motion.
• Application to methanol shows significant improvement in describing torsional frequency using curvilinear coordinates.
• The curvilinear formulation of fast-VSCF/VCI doubles the speed and triples the accuracy.

Conclusions:

• The new computational approach provides a robust and efficient tool for studying molecular vibrations, particularly large-amplitude motions.
• Curvilinear coordinates are essential for accurate VSCF/VCI descriptions of torsional motions.
• This method offers a significant advancement in computational molecular spectroscopy.