A general spin-complete spin-flip configuration interaction method.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate electronic structure calculations are crucial for understanding chemical phenomena.
• Existing methods may struggle with systems exhibiting significant static correlation, such as diradicals or bond breaking processes.
• The development of versatile computational methods is essential for advancing chemical research.

Purpose of the Study:

• Introduce a novel, general spin-correct spin-flip configuration interaction (SF-CI) method.
• Extend the occupation-restricted multiple active spaces (ORMAS) CI method to incorporate spin-flip excitations.
• Provide a flexible computational tool for studying diverse chemical systems with varying multiplicities.

Main Methods:

• Developed the spin-flip occupation-restricted multiple active spaces (SF-ORMAS) method within the GAMESS computational chemistry package.
• Employed a single reference, high-spin restricted open-shell determinant as the starting point.
• Incorporated spin-flipped excitations to generate wave functions of desired multiplicity.
• Included dynamic correlation effects via perturbation theory.

Main Results:

• Demonstrated the efficacy of SF-ORMAS for single and multiple bond breaking scenarios.
• Successfully characterized diradical systems using the new methodology.
• Applied the method to calculate vertical excitations in linear alkenes.
• Computed the singlet-triplet energy gap for silicon trimer.

Conclusions:

• The SF-ORMAS method offers a versatile and accurate approach for electronic structure calculations.
• The flexibility in active space design allows tailored application to specific chemical problems.
• The method successfully addresses challenges in systems with significant static correlation and varying spin states.