Generalization of Block-Localized Wave Function for Constrained Optimization of Excited Determinants

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• The block-localized wave function (BLW) method is crucial for analyzing chemical bonding and intermolecular interactions via energy decomposition.
• BLW relies on constraining molecular orbitals (MOs) to specific basis functions within defined blocks for localization.

Purpose of the Study:

• To introduce a generalized block-localized orbital (GBLO) method that allows for both localized and delocalized MOs within orbital-block definitions.
• To demonstrate the flexibility and improved accuracy of the GBLO method through tailored optimization constraints.

Main Methods:

• The GBLO method is applied to three distinct chemical systems: constrained polarization response in a water dimer, C-C bond rotation in ethene, and double electron excited states.
• Multistate density functional theory (MS-DFT) combined with a minimal active space (MAS) approach is used for variational optimization.
• The MAS includes charge-constrained and excited determinant configurations optimized via the GBLO method.

Main Results:

• The GBLO method, when including delocalized MOs in configurational blocks, significantly reduces computational errors compared to traditional physical block localization.
• Computed ground- and excited-state energies show excellent agreement with experimental data.
• Results are also consistent with high-level multireference configuration interaction (MRCI) calculations.

Conclusions:

• The GBLO method offers a more accurate and flexible approach for studying electronic structures, chemical bonding, and excited states.
• Incorporating delocalized MOs is key to improving the precision of energy decomposition analyses and electronic structure calculations.