Multiconfiguration Pair-Density Functional Theory Spectral Calculations Are Stable to Adding Diffuse Basis Functions.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Spectroscopy

Background:

• Time-dependent Kohn-Sham density functional theory (TD-KS-DFT) is widely used for electronic excitation spectra of large systems.
• TD-KS-DFT often suffers from artificially lowered higher-energy states, impacting even the lowest energy excited states.
• Accurate calculation of electronic excitation energies is crucial for understanding molecular properties and reactions.

Purpose of the Study:

• To investigate the performance of multiconfiguration pair-density functional theory (MC-PDFT) for calculating electronic excitation spectra.
• To assess whether MC-PDFT avoids the artificial lowering of higher-energy states observed in TD-KS-DFT.
• To compare MC-PDFT with the tPBE functional against established methods like CASPT2 and TD-KS-DFT with PBE and PBE0 functionals.

Main Methods:

• Calculated the lowest energy spin-conserving excited state for atoms (H to K) and formaldehyde.
• Employed multiconfiguration pair-density functional theory (MC-PDFT) with the tPBE on-top density functional.
• Compared results with complete active space second-order perturbation theory (CASPT2) and TD-KS-DFT using PBE and PBE0 functionals.

Main Results:

• MC-PDFT with the tPBE functional did not exhibit the artificial lowering of higher-energy states.
• For atomic systems, MC-PDFT with tPBE showed improved mean unsigned error (MUE) from 0.42 to 0.40 eV with diffuse functions.
• TD-KS-DFT with PBE and PBE0 functionals showed drastically increased MUEs (0.74 to 2.49 eV and 0.45 to 1.47 eV, respectively) for atomic systems.

Conclusions:

• MC-PDFT offers a reliable alternative to TD-KS-DFT for calculating electronic excitation spectra, particularly for avoiding state-lowering artifacts.
• The tPBE functional within MC-PDFT demonstrates robust accuracy for atomic systems, outperforming standard TD-KS-DFT functionals.
• The addition of diffuse functions further refines the accuracy of MC-PDFT calculations.