Relativistic quasidegenerate perturbation theory with four-component general multiconfiguration reference functions

Area of Science:

• Quantum Chemistry
• Computational Physics
• Relativistic Quantum Mechanics

Background:

• Accurate electronic structure calculations are crucial for understanding molecular and atomic properties.
• Existing relativistic methods can be computationally expensive or lack accuracy for certain systems.
• General multiconfiguration (GMC) reference functions offer advantages in computational efficiency and applicability.

Purpose of the Study:

• To develop and implement a relativistic quasidegenerate perturbation theory (QDPT) using general multiconfiguration (GMC) reference functions.
• To provide a computationally efficient and accurate method for systems with heavy elements.
• To assess the performance of the new method for molecular potential energy curves and atomic term energies.

Main Methods:

• Development of relativistic quasidegenerate perturbation theory (QDPT).
• Utilizing general multiconfiguration (GMC) reference functions.
• Application to potential-energy curves of I(2) and Sb(2) molecules, excitation energies of CH(3)I, and lowest terms of C, Si, and Ge atoms.

Main Results:

• The developed relativistic QDPT with GMC references provides a balanced description of potential-energy curves.
• The method yields accurate transition energies for systems containing heavy elements.
• Results show significant improvement compared to the reference function (active space configuration interaction) level.

Conclusions:

• Relativistic QDPT with GMC references is a powerful and efficient tool for electronic structure calculations involving heavy elements.
• The method retains the advantages of nonrelativistic GMC reference QDPT, including broad applicability and reduced computational cost.
• This approach offers a superior alternative to standard reference function methods for complex systems.