Low-lying excited states by constrained DFT.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Calculating low-lying excited states of molecular systems is crucial for understanding photochemistry and photophysics.
• Existing methods like time-dependent Density Functional Theory (TDDFT) and delta Self-Consistent Field (ΔSCF) have limitations, including variational collapse and computational cost.

Purpose of the Study:

• To introduce and validate a novel variational method, eXcited Constrained Density Functional Theory (XCDFT), for computing excited states.
• To assess the accuracy, applicability, and efficiency of XCDFT compared to established methods.

Main Methods:

• Developed XCDFT based on Constrained Density Functional Theory (CDFT).
• Excited states are calculated by constraining a specific electron population (N_c = 1.0) in virtual orbitals.
• Tested XCDFT on 15 molecules for excitation energies, challenging processes (conical intersections), and condensed-phase systems (solvatochromic shifts).

Main Results:

• XCDFT demonstrates accuracy comparable to TDDFT for excitation energies.
• The method avoids variational collapse issues inherent in the ΔSCF approach.
• XCDFT successfully reproduces energy surfaces with conical intersections and solvatochromic shifts, outperforming TDDFT in some cases.
• XCDFT shows competitive performance in terms of speed, convergence, and accuracy.

Conclusions:

• XCDFT is a robust and accurate variational method for calculating excited states in molecular systems.
• It offers a compelling alternative to TDDFT and ΔSCF, particularly for systems with complex electronic structures or in condensed phases.
• The method's efficiency and reliability make it suitable for a wide range of computational chemistry applications.