We report detailed evidence on the crossing and avoided crossing between the (X) over tilde(1)A' and 2(1)A' potential energy surfaces of HCN from ab initio full valence complete active space calculations. The role of three-body effects on explaining the crossing seam and the implications of the latter on reaction dynamics are also briefly discussed. (C) 1997 American Institute of Physics. [S0021-9606(97)00446-7].