Dynamic correlation for non-orthogonal reference states: Improved perturbational and variational methods.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Standard methods use orthogonal molecular orbitals, limiting wave function compactness.
• Molecules with partly occupied atomic orbitals, like transition metal complexes, require advanced methods.
• Efficient dynamic correlation methods are crucial for accurate molecular modeling.

Purpose of the Study:

• To develop efficient dynamic correlation methods using non-orthogonal active orbitals.
• To improve the non-orthogonal internally contracted perturbation theory approach.
• To accurately model systems with complex electronic structures.

Main Methods:

• Employing a reference state with non-orthogonal active orbitals.
• Utilizing the Dyall Hamiltonian with two-electron interactions in the active space as the zero-order operator.
• Calculating third-order energy corrections and including excitations within the active orbital space.

Main Results:

• Developed an improved non-orthogonal internally contracted perturbation theory.
• Successfully modeled the nitrogen molecule and the chromium dimer.
• Achieved potential curves for the chromium dimer comparable to larger reference wave functions.

Conclusions:

• The enhanced methods provide more compact wave functions.
• The improvements accurately describe dynamic correlation in challenging systems.
• This approach offers a computationally efficient way to model complex molecules.