Strongly contracted canonical transformation theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Canonical Transformation (CT) theory models dynamic correlation in multireference systems.
• Large active spaces present challenges, including the intruder state problem.
• Previous methods like overlap matrix truncation become computationally infeasible for large systems.

Purpose of the Study:

• To address the intruder state problem in CT theory for large active spaces.
• To introduce and evaluate strongly and weakly contracted excitation operators as alternatives.
• To enable accurate modeling of dynamic correlation in complex molecular systems.

Main Methods:

• Developed and applied strongly and weakly contracted excitation operators within CT theory.
• Evaluated performance against complete active space second order perturbation theory (CASPT2) and Davidson-corrected multireference configuration interaction (MRCI+Q).
• Combined strongly contracted CT theory with orbital-optimized density matrix renormalization group (ODMRG) theory.

Main Results:

• The proposed excitation operators effectively manage intruder states in CT theory.
• Accurate calculations were performed for H2O, N2, and NiO molecules.
• The singlet-triplet gap of free base porphin was evaluated using a large active space (24 orbitals).

Conclusions:

• Strongly and weakly contracted excitation operators offer a viable solution to the intruder state problem in CT theory.
• CT theory, particularly when combined with ODMRG, can model dynamic correlation in very large active spaces.
• This approach is crucial for studying complex systems where traditional methods fail.