CASCICASSCF

CASCICASSCF (Complete Active Space Configuration Interaction with Complete Active Space Self-Consistent Field) is a computational quantum chemistry method used to accurately describe electronic structures of molecules, particularly those with strong electron correlation effects. It is an extension of the CASSCF (Complete Active Space Self-Consistent Field) methodology, integrating configuration interaction techniques within a flexible active space framework.

The primary goal of CASCICASSCF is to improve the accuracy of electronic structure calculations by combining the benefits of CASSCF with configuration interaction (CI) methods. In the CASSCF approach, a specified set of orbitals, known as the active space, is used to generate all possible electronic configurations, capturing static correlation effects. CASCICASSCF further refines this by explicitly including CI calculations within the active space, enabling a more comprehensive treatment of correlation effects, including dynamic correlation.

This method is particularly useful for studying molecules with near-degenerate electronic states, bond-breaking processes, and transition metal complexes, where traditional single-reference methods often fall short. By systematically accounting for multiple electronic configurations, CASCICASSCF provides insight into complex electronic phenomena and improves the accuracy of predicted spectroscopic and chemical properties.

CASCICASSCF implementations typically require significant computational resources due to the large configuration spaces involved. As such, it is often employed in research settings for detailed electronic structure analysis rather than routine calculations. Its development and application contribute significantly to the fields of theoretical and computational chemistry, especially in understanding complex molecules with correlated electrons.