A DFT/MRCI Hamiltonian parameterized using only ab initio data: I. valence excited states

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of electronic excitation energies is crucial for understanding molecular properties and reactivity.
• Existing methods, such as density functional theory and multi-reference configuration interaction (DFT/MRCI), have limitations in describing diverse electronic excitations.
• Parameterization of these methods often relies on specific datasets, potentially limiting their general applicability.

Purpose of the Study:

• To develop and validate a new DFT/MRCI Hamiltonian (QE8) parameterized using benchmark ab initio vertical excitation energies.
• To investigate the impact of the exchange-correlation (XC) functional choice on the accuracy of DFT/MRCI calculations.
• To assess the performance of approximate DFT/MRCI methods (p-DFT/MRCI and DFT/MRCI(2)) with the new Hamiltonian.

Main Methods:

• Development of a novel DFT/MRCI Hamiltonian (QE8) using the QUEST database for ab initio vertical excitation energies.
• Selection of a new XC functional, QTP17, for improved description of core and valence excitations.
• Validation of the QE8 Hamiltonian against benchmark quantum chemistry computations.

Main Results:

• The QE8 Hamiltonian achieves a mean absolute error of 0.16 eV for a wide range of electronic excitations.
• QE8 significantly enhances the accuracy of DFT/MRCI for doubly excited states.
• Approximate methods (p-DFT/MRCI and DFT/MRCI(2)) using QE8 show quantitative agreement with the full DFT/MRCI method, with a mean difference of 0.01 eV.

Conclusions:

• The new QE8 Hamiltonian provides a more accurate and balanced description of vertical excitation energies compared to previous DFT/MRCI formulations.
• The choice of XC functional is critical for achieving high accuracy in DFT/MRCI calculations.
• Fast approximate DFT/MRCI methods are reliable when parameterized with the accurate QE8 Hamiltonian.