State-Interaction Pair-Density Functional Theory Can Accurately Describe a Spiro Mixed Valence Compound

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Mixed-valence compounds with strong electronic couplings present significant challenges for multireference electronic structure theory.
• Previous studies indicated that third-order multireference perturbation theory (MRPT) or specialized orbital averaging schemes were necessary for accurate energy surfaces of these systems.
• The prototype mixed-valence compound studied is the 2,2',6,6'-tetrahydro-4 H,4' H-5,5'-spirobi[cyclopenta[ c]pyrrole] cation.

Purpose of the Study:

• To investigate the efficacy of second-order MRPT methods for modeling mixed-valence compounds.
• To evaluate the performance of state-interaction pair-density functional theory (SI-PDFT) for these challenging systems.
• To identify computationally efficient and accurate methods for describing multireference systems.

Main Methods:

• Calculation of the Fock operator for the zeroth-order Hamiltonian using a state-averaged density matrix within second-order MRPT frameworks (CASPT2, MS-CASPT2, XMS-CASPT2).
• Application of state-interaction pair-density functional theory (SI-PDFT) to the mixed-valence compound near an avoided crossing.
• Comparison of the results from different second-order MRPT methods and SI-PDFT against established theoretical approaches.

Main Results:

• Second-order MRPT methods, when employing a state-averaged density matrix, yield accurate energy surfaces for the model mixed-valence compound.
• State-interaction pair-density functional theory (SI-PDFT) successfully avoids the unphysical behavior observed in other second-order MRPT methods near the avoided crossing.
• SI-PDFT demonstrates promising accuracy for this prototype system, suggesting its potential for larger and more complex molecular structures.

Conclusions:

• Second-order MRPT methods can accurately describe challenging mixed-valence systems when implemented with a state-averaged density matrix.
• State-interaction pair-density functional theory (SI-PDFT) emerges as a robust and computationally efficient method for studying mixed-valence compounds.
• The findings encourage the application of SI-PDFT to larger and more complex multireference systems due to its favorable cost-performance ratio.