Molecular excited states from the SCAN functional

ABSTRACT

The performance of the strongly constrained and appropriately normed (SCAN) meta-generalised gradient approximation exchange–correlation functional is investigated for the calculation of time-dependent density-functional theory molecular excitation energies of local, charge-transfer and Rydberg character, together with the excited potential energy curve in H₂. The SCAN results frequently resemble those obtained using a global hybrid functional, with either a standard or increased fraction of exact orbital exchange. For local excitations, SCAN can exhibit significant triplet instability problems, resulting in imaginary triplet excitation energies for a number of cases. The Tamm–Dancoff approximation offers a simple approach to improve the situation, but the excitation energies are still significantly underestimated. Understanding the origin of these (near)-triplet instabilities may provide useful insight into future functional development.

KEYWORDS:

• Density-functional theory
• exchange–correlation functionals
• time-dependent DFT
• excited states
• triplet instabilities

Acknowledgments

Michael J. G. Peach thanks Lancaster University, and the Joy Welch Educational Charitable Trust for financial support. David J. Tozer thanks Durham University for financial support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Lancaster University; the Joy Welch Educational Charitable Trust; Durham University.