Spin-Density Functional Regularization for Singlet Diradicals.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Broken-symmetry density functional theory (DFT) is widely used for static correlation in singlet diradicals.
• This method can fail for complex electronic structures due to artificial spin symmetry breaking.
• Singlet diradicals arise from (quasi-)degenerate frontier orbitals.

Purpose of the Study:

• To develop a method to correct errors from artificial symmetry breaking in broken-symmetry DFT.
• To improve the accuracy of energy calculations for systems with complex electronic structures.

Main Methods:

• Introduction of a spin-density functional regularization (SR) approach.
• Integration of SR within the framework of partition density functional theory (PDFT).
• Application to the automerization of cyclobutadiene, a system with a singlet diradical transition state.

Main Results:

• The SR-PDFT approach effectively corrects errors caused by artificial symmetry breaking.
• Conventional broken-symmetry DFT calculations underestimate the automerization transition state energy for cyclobutadiene.
• SR-PDFT yields chemically accurate barrier heights for the cyclobutadiene automerization reaction.

Conclusions:

• The SR-PDFT method provides a robust way to handle static correlation in singlet diradicals.
• This approach overcomes limitations of conventional broken-symmetry DFT for complex systems.
• Accurate prediction of reaction barrier heights is achieved, demonstrating the method's efficacy.