R2022: A DFT/MRCI Ansatz with Improved Performance for Double Excitations.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• The combined density functional theory and multireference configuration interaction (DFT/MRCI) method has been a valuable tool in computational chemistry.
• Previous formulations of DFT/MRCI faced challenges in accurately describing doubly excited electronic states, particularly the ¹Δ_g state in diatomic carbon.

Purpose of the Study:

• To present a reformulated DFT/MRCI method with improved accuracy for electronic structure calculations.
• To address the limitations of previous DFT/MRCI formulations in handling doubly excited states.

Main Methods:

• Derivation of correction terms for an effective Hamiltonian using ab initio matrix elements.
• Introduction of split parameters for intra- and interorbital interactions to capture underlying physics.
• Application and validation of the new formulation on diatomic carbon, polyacenes, and mini-β-carotenoids.

Main Results:

• The reformulated DFT/MRCI method successfully describes problematic doubly excited states, such as the ¹Δ_g state in diatomic carbon.
• Accurate calculations of ¹L_a and ¹L_b states in polyacenes and ¹A_u and ¹A_g states in mini-β-carotenoids demonstrate the method's superiority.
• Statistical analysis confirms that the new formulation retains all advantages of previous DFT/MRCI Hamiltonians.

Conclusions:

• The presented reformulation of DFT/MRCI offers a more concise and accurate description of electronic interactions.
• This improved method provides superior results compared to former effective Hamiltonians.
• The new formulation is proposed as the new standard for DFT/MRCI calculations in computational chemistry.