Vibrational coupled cluster theory with full two-mode and approximate three-mode couplings: the VCC[2pt3] model

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Molecular Physics

Background:

• Vibrational coupled cluster (VCC) theory is crucial for calculating molecular vibrational energy levels.
• Current VCC models vary in their treatment of mode couplings and excitation spaces, impacting accuracy and computational cost.
• Accurate prediction of vibrational energy levels is essential for understanding molecular dynamics and spectroscopy.

Purpose of the Study:

• To introduce a novel VCC model incorporating two-mode couplings and an approximate treatment of three-mode couplings.
• To develop an iterative VCC method utilizing response theory for excitation energy calculations.
• To assess the computational scaling and accuracy of the proposed VCC model.

Main Methods:

• Development of a VCC model including all two-mode couplings and approximate three-mode couplings.
• Application of perturbational analysis for the three-mode coupling approximation.
• Iterative VCC calculations and VCC response theory for excitation energies.
• Utilizing grid-based potential energy surfaces from CCSD(T) calculations for benchmark studies.

Main Results:

• The proposed VCC model demonstrates efficient computational scaling, proportional to the cube of the number of vibrational modes.
• Benchmark calculations on small molecules (3-4 atoms) show encouraging accuracy.
• Accurate calculation of fundamental vibrational frequencies for ethylene oxide using a CCSD(T)-derived potential energy surface.

Conclusions:

• The new VCC model provides a computationally efficient and accurate approach for determining molecular vibrational energy levels.
• The method's scaling is comparable to VCC models with only two-mode couplings, making it suitable for larger systems.
• This work advances the capability of VCC theory for spectroscopic and dynamical studies of molecules.