Portions of the S1[1A(2 1A)]–S0[1A(1 1A)] seam of conical intersection relevant to the internal conversion S1→S0 of photoexcited isocyanic acid HNCO are analyzed. The topography of the potential energy surfaces, and the derivative coupling, in the vicinity of a conical intersection is described in terms of four conical parameters. These parameters are also used to obtain a local diabatic representation that removes the singularity in the derivative coupling. Continuity is achieved through the use of a recently described orthogonalization procedure. The conical parameters demonstrate that the double cones of concern are significantly tilted, which has important implications for the nuclear dynamics.

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