Reduced-dimensional vibrational models of the water dimer

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Quantum Mechanics

Background:

• Accurate calculation of vibrational energy levels and transitions is crucial for understanding molecular dynamics.
• The water dimer presents a complex system due to coupled intra- and intermolecular vibrations.
• Previous models often face challenges with computational cost or approximations in the kinetic energy operator and potential energy surface.

Purpose of the Study:

• To develop a finite-basis representation model for the vibrational Hamiltonian of the water dimer.
• To accurately compute intramolecular OH stretch and HOH bend transitions.
• To investigate the impact of reduced dimensionality and approximations on accuracy.

Main Methods:

• Development of a vibrational Hamiltonian model using internal coordinates and a finite-basis representation.
• Application of many-mode, low-order expansions for kinetic energy and potential energy surface (PES).
• Utilized polyad truncations and energy ceilings for basis set control; performed reduced-dimensional calculations and full 12-dimensional computations using symmetry.

Main Results:

• The model accurately predicts experimentally observed intramolecular fundamental transitions of the water dimer (H2 16O)2.
• Wavenumber shifts due to intra- and intermolecular mode coupling were investigated.
• Computationally inexpensive reduced-dimensional calculations can accurately predict intramolecular transition wavenumbers.

Conclusions:

• The developed model provides an accurate and efficient method for calculating water dimer vibrational spectra.
• Reduced-dimensional approaches, when carefully applied, can yield results comparable to full-dimensional calculations.
• This work guides future effective-Hamiltonian treatments for similar molecular systems.