2D-block geminals: Guidelines to choose effective excitations

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The electronic Schrödinger equation is fundamental to molecular behavior.
• Antisymmetrized product of strongly orthogonal geminals (APSG) is a known method.
• APSG has limitations including 'strong orthogonality' and 'seniority zero' restrictions.

Purpose of the Study:

• Introduce a new family of geminal Ansätze: antisymmetrized product of 2D-block geminals (AP2D-BG).
• Address limitations of APSG by lifting 'strong orthogonality' and 'seniority zero' restrictions.
• Develop practical guidelines for reducing computational complexity in AP2D-BG.

Main Methods:

• Developed AP2D-BG by building upon APSG.
• Investigated diatomic molecules to test the method.
• Formulated simple guidelines to drastically reduce the number of excitations.
• Explored strategies for partitioning orbital space into 1D- and 2D-blocks.

Main Results:

• Demonstrated that AP2D-BG significantly lowers electronic energy in variational calculations.
• Identified a simple, universally successful strategy for selecting block types.
• Observed that optimal geminals correlate with classical Lewis structures.
• Achieved high-quality potential energy curves (PECs) for diatomic molecules.

Conclusions:

• AP2D-BG offers a practical and accurate approach to solving the electronic Schrödinger equation.
• The developed guidelines make the AP2D-BG method computationally feasible.
• The method shows significant improvement over APSG, particularly in PEC calculations.