On the triplet instability in TDDFT
Michael J. G. Peach, Neil Warner, David J. Tozer, Mol. Phys. 111 1271–1274 (2013)
This paper was part of a special issue honouring Professor Trygve Helgaker, of the University of Oslo. The work is a follow-up to some of my previous work on the triplet instability and how it can affect TDDFT excitation energies. In particular, this considers two aspects of this problem. One is the usefulness of using the stability of the Hartree–Fock single determinant wavefunction as an indication of when excitations will be unreliably described by conventional functionals in TDDFT. The second aspect involves the B97-2 functional, which displays a surprisingly small triplet instability problem, given the amount of exact exchange that the functional contains.