Optimizing Conical Intersections by SpinFlip Density Functional Theory: Application to Ethylene
ABSTRACT Conical intersections (CIs) of ethylene have been successfully determined using spinflip density functional theory (SFDFT) combined with a penaltyconstrained optimization method. We present in detail three structures, twistedpyramidalized, hydrogenmigrated, and ethylidene CIs. In contrast to the linear response timedependent density functional theory, which predicts a purely twisted geometry without pyramidalization as the S(1) global minimum, SFDFT gives a pyramidalized structure. Therefore, this is the first correct optimization of CI points of twisted ethylene by the DFT method. The calculated energies and geometries are in good agreement with those obtained by the multireference configuration interaction (MRCI) method and the multistate formulation of secondorder multireference perturbation theory (MSCASPT2).

 "This has lead to various attempts to include 2h2p states in TDDFT. One partial solution was given by spinflip TDDFT [37] [38] which describes some states which are 2h2p with respect to the ground state by beginning with the lowest triplet state and including spinflip excitations [39] [40] [41] [42]. However , spinflip TDDFT does not provide a general way to include double excitations. "
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ABSTRACT: Almost all timedependent densityfunctional theory (TDDFT) calculations of excited states make use of the adiabatic approximation, which implies a frequencyindependent exchangecorrelation kernel that limits applications to onehole/oneparticle states. To remedy this problem, Maitra et al.[J.Chem.Phys. 120, 5932 (2004)] proposed dressed TDDFT (DTDDFT), which includes explicit twohole/twoparticle states by adding a frequencydependent term to adiabatic TDDFT. This paper offers the first extensive test of DTDDFT, and its ability to represent excitation energies in a general fashion. We present DTDDFT excited states for 28 chromophores and compare them with the benchmark results of Schreiber et al.[J.Chem.Phys. 128, 134110 (2008).] We find the choice of functional used for the ATDDFT step to be critical for positioning the 1h1p states with respect to the 2h2p states. We observe that DTDDFT without HF exchange increases the error in excitations already underestimated by ATDDFT. This problem is largely remedied by implementation of D TDDFT including HartreeFock exchange.Chemical Physics 12/2010; 391(1). DOI:10.1016/j.chemphys.2011.03.019 · 2.03 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the ability of spinflip time dependent density functional theory (TDDFT) to uniformly describe excited states of single, double, and mixed excitation character in closedshell molecular systems, using the polyene oligomers as a primary test case. The results of comparison between conventional and spinflip TDDFT and with correlated ab initio methods indicate that spinflip TDDFT provides a more consistent description of the ordering and relative positions of the excited states than conventional TDDFT provided a suitable exchangecorrelation functional is used in the calculations. It is found that spinflip TDDFT provides a physically appealing picture of excitation processes which involve one or two electrons, as it captures their most important features and facilitates a more uniform description of excited states with different character. This makes spinflip TDDFT a promising approach for general modeling of excited states and spectra of medium and large size molecules, which exhibit lowlying excited states with strong double excitation character.The Journal of Chemical Physics 09/2010; 133(11):114104. DOI:10.1063/1.3479401 · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Under the usual assumption of noninteracting vrepresentability, densityfunctional theory (DFT) together with timedependent DFT (TDDFT) provide a formally exact singlereference method suitable for the theoretical description of the electronic excitedstates of large molecules, and hence for the description of excitedstate potential energy surfaces important for photochemistry. The quality of this singlereference description is limited in practice by the need to use approximate exchangecorrelation functionals. In particular it is far from clear how well approximations used in contemporary practical TDDFT calculations can describe funnel regions such as avoided crossings and conical intersections. These regions typically involve biradicallike structures associated 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 spinpreserving (SP) TDDFT, it is present to some extent in spinflip (SF) TDDFT. We report our tests of WangZiegler noncollinear SFTDDFT within the TammDancoff approximation for describing the avoided crossing in the C(2v) CC ringopening reaction of oxirane and for describing the conical intersection relevant for the more physical asymmetric CO ringopening reaction of oxirane. Comparisons are made with complete active space selfconsistent field and quantum Monte Carlo benchmark results from two previous papers on the subject [J. Chem. Phys., 2007, 127, 164111; ibid 129, 2008, 124108]. While the avoided crossing in the C(2v) pathway is found to be reasonably well described, the method was found to be only partially successful for the conical intersection (CX) associated with the physically more important asymmetric pathway. The origin of the difficulties preventing the noncollinear SFTDDFT method from giving a completely satisfactory description of the CX was traced back to the inability of SFTDDFT based upon a single triplet reference state to correlate all potentially relevant configurations involving not just two but three nearly degenerate orbitals (n, σ(CO), and σ(CO)(*)). This article is also the first report of our implementation of SFTDDFT within the deMon2k program.Physical Chemistry Chemical Physics 10/2010; 12(39):1281125. DOI:10.1039/c0cp00273a · 4.20 Impact Factor