Département de chimie moléculaire

Under the usual assumption of noninteracting v-representability, d.-functional theory (DFT) together with time-dependent DFT (TDDFT) provide a formally exact single-ref. method suitable for the theor. description of the electronic excited-states of large mols., and hence for the description of excited-state potential energy surfaces important for photochem. The quality of this single-ref. description is limited in practice by the need to use approx. exchange-correlation functionals. In particular it is far from clear how well approxns. used in contemporary practical TDDFT calcns. can describe funnel regions such as avoided crossings and conical intersections. These regions typically involve biradical-like structures assocd. with bond breaking and conventional wisdom would seem to suggest the need to introduce explicit double excitation character to describe these structures. Although this is lacking in ordinary spin-preserving (SP) TDDFT, it is present to some extent in spin-flip (SF) TDDFT. We report our tests of Wang-Ziegler noncollinear SF-TDDFT within the Tamm-Dancoff approxn. for describing the avoided crossing in the C2v CC ring-opening reaction of oxirane and for describing the conical intersection relevant for the more phys. asym. CO ring-opening reaction of oxirane. Comparisons are made with complete active space SCF and quantum Monte Carlo benchmark results from two previous papers on the subject [J. Chem. Phys., 2007, 127, 164111; ibid129, 2008, 124108]. While the avoided crossing in the C2v pathway is found to be reasonably well described, the method was found to be only partially successful for the conical intersection (CX) assocd. with the phys. more important asym. pathway. The origin of the difficulties preventing the noncollinear SF-TDDFT method from giving a completely satisfactory description of the CX was traced back to the inability of SF-TDDFT based upon a single triplet ref. state to correlate all potentially relevant configurations involving not just two but three nearly degenerate orbitals (n, $\sigma$CO, and ). This article is also the first report of our implementation of SF-TDDFT within the deMon2k program. [on SciFinder(R)]