Multi-reference vibration correlation methods.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Accurate calculation of state-specific vibrational energies is crucial for understanding molecular systems.
• Strong anharmonic resonances pose significant challenges for standard computational methods.
• The vibrational multi-configuration self-consistent field (VMCSCF) approximation has limitations in these scenarios.

Purpose of the Study:

• To develop and present advanced state-specific vibration correlation methods.
• To enable accurate calculation of state energies in systems with strong anharmonic resonances.
• To investigate relaxation effects and compare different modal basis approaches.

Main Methods:

• Implementation of variational multi-reference configuration interaction (MRCI) approaches.
• Application of approximate 2nd order vibrational multi-reference perturbation theory (VMP2).
• Development of a configuration selection scheme to accelerate the variational approach.

Main Results:

• The presented methods accurately compute state energies for systems with strong anharmonic resonances.
• The variational approach with configuration selection shows negligible deviations in results.
• Detailed investigation of relaxation effects and VMCSCF modal basis performance was conducted.

Conclusions:

• The developed state-specific vibration correlation methods offer significant improvements over VMCSCF.
• These methods are effective for systems exhibiting strong anharmonic resonances.
• Benchmark calculations on the propargyl cation and cis-diazene validate the approaches.