A second-order multi-reference perturbation method for molecular vibrations.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Quantum Chemistry

Background:

• Strong correlation in vibrational structure theory presents significant computational challenges.
• Accurate treatment of molecular vibrations is crucial for understanding chemical dynamics and spectroscopy.
• Existing methods often struggle with large anharmonic couplings and state resonances.

Purpose of the Study:

• To introduce a general multi-reference framework for treating strong correlation in vibrational structure theory.
• To develop a novel perturbation correction for weak correlation between vibrational modes.
• To implement and apply these methods for accurate vibrational energy calculations.

Main Methods:

• Vibrational Active Space Self-Consistent Field (VASSCF) approach for selecting active configurations.
• State-specific second-order multi-configurational perturbation (VASPT2) correction for weak correlation.
• Implementation within the DYNAMOL dynamics package, handling up to four-body Hamiltonian coupling terms.

Main Results:

• The VASPT2 method accurately and efficiently treats strong correlation, including large anharmonic couplings and resonances.
• Pilot application to trans-formic acid using a new analytic potential shows high accuracy.
• Computed fundamental transition energies are within 9 cm(-1) RMSD of experimental values.

Conclusions:

• The VASPT2 method provides a robust framework for accurate vibrational structure calculations.
• It achieves accuracy comparable to high-level correlated electronic structure methods.
• The DYNAMOL package facilitates the application of these advanced methods.