Overcoming Low Orbital Overlap and Triplet Instability Problems in TDDFT
Michael J. G. Peach, David J. Tozer, J. Phys. Chem. A, 116, 9783–9789, 2012.
Through the development of a new benchmark excitation set using EOM-CCSD excited states and the subsequent assessment of several representative exchange–correlation functionals, this paper brings together our work on two topics: orbital overlap and triplet instability problems in TDDFT. The first problem we quantify using Lambda, as defined in the 2008 Lambda paper. The second we quantify by computing triplet stability measures, as we did previously in our 2011 paper on triplet excited states. In this new work, we extend the application of Lambda to triplet excited states, and go on to show the implicit connection between Lambda and triplet instabilities: triplet stabilities must be associated with high orbital overlap (often caused by excitations between highly localised orbitals centred on the same area of a molecule, such as the low-lying excitations in aromatic systems). It is therefore not possible to have simultaneous failures of orbital overlap and triplet stability for the same excited state, although both problems can be seen in the same molecule.
Intuitively, the Tamm–Dancoff approximation also has a larger effect as the overlap increases (for small overlap, conventional and TDA excitation energies are identical). We illustrate that particularly high accuracy can be achieved using a combined Coulomb-attenuated Tamm–Dancoff approach.
Further information, including details of subsequent work in this area, can be found on the TDDFT diagnostic research page. For the abstract, and access to the full text, see below.