The barrier to linearity in the HNCO molecule has been determined from ab initio quantum mechanical computation to be 4.8 ± 0.5 kcal/mole. Careful attention is paid to the effects of geometry optimization, correlation effects, and the magnitudes of errors which can occur in the computations. A useful procedure for evaluating the accuracy of estimated optimized geometries from correlated wavefunctions is presented. The shape of the HNC bending potential, allowing full geometrical relaxation, has been determined. Its curvature at the minimum gives an effective bending force constant of 0.24 mdyne Å/rad2. © 1978.