CAS without SCF-Why to use CASCI and where to get the orbitals

Area of Science:

• Computational chemistry
• Quantum chemistry
• Theoretical chemistry

Background:

• Complete active space self-consistent field (CASSCF) is widely used for molecular electronic structure.
• CASSCF can describe ground and excited states but is computationally demanding and complex.
• Hartree-Fock approximation is simpler but less accurate for complex systems.

Purpose of the Study:

• To present complete active space configuration interaction (CASCI) as a viable alternative to CASSCF.
• To highlight CASCI's advantages for describing electronic excited states.
• To discuss CASCI's suitability for nanomaterials and the need for improved orbital definitions.

Main Methods:

• Discussion of CASCI methods as alternatives to CASSCF.
• Exploration of orbital computation schemes for CASCI.
• Analysis of CASCI's application to electronic excited states and nanomaterials.

Main Results:

• CASCI can offer qualitative advantages over CASSCF, not just be an approximation.
• Insights gained from experimenting with different orbital computation schemes for CASCI.
• CASCI is well-suited for the electronic structure of nanomaterials.

Conclusions:

• CASCI presents a powerful alternative to CASSCF, particularly for excited states.
• Developing new orbital definition methods is crucial for efficient and accurate CASCI.
• CASCI, coupled with low-scaling approaches, can address multireference system challenges.