Portions of the S-1[(1)A " (2 (1)A)]-S-0[(1)A'(1 (1)A)] seam of conical intersection relevant to the internal conversion S-1-->S-0 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. (C) 2001 American Institute of Physics.