A perturbative approximation to DFT/MRCI: DFT/MRCI(2).

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of electronic energies is crucial for understanding molecular properties and reactions.
• Traditional methods like Density Functional Theory and Multireference Configuration Interaction (DFT/MRCI) can be computationally expensive, limiting their application to larger systems.

Purpose of the Study:

• To develop a computationally efficient approximation to the DFT/MRCI method.
• To maintain high accuracy in calculating electronic energies, particularly excitation energies.

Main Methods:

• Introduction of a perturbative approximation, DFT/MRCI(2), to the combined DFT/MRCI approach.
• Application of quasi-degenerate perturbation theory (QDPT) and Epstein-Nesbet partitioning to the DFT/MRCI Hamiltonian matrix.
• Utilizing QDPT to avoid diagonalization of the large DFT/MRCI Hamiltonian, employing a smaller effective Hamiltonian instead.

Main Results:

• The DFT/MRCI(2) method offers significant savings in computational cost, orders of magnitude lower than canonical DFT/MRCI.
• Achieved excellent accuracy for excitation energies, with a root mean squared deviation of less than 0.03 eV compared to DFT/MRCI.
• Validated on an extensive test set of organic molecules.

Conclusions:

• The DFT/MRCI(2) approximation provides a computationally tractable and highly accurate alternative for electronic structure calculations.
• This method enables more efficient and accessible high-level quantum chemical computations for complex molecular systems.