Ab initio electronic structure theory has been used to determine the more important features of the potential energy surface for the simple isomerization reaction HNC → HCN. Extended basis sets were used in conjunction with both self-consistent-field (SCF) and configuration interaction (CI) wavefunctions. For nonlinear or Cs geometrical arrangements of the three atoms, the CI included 11 735 configurations, i.e., all single and double excitations. This large scale CI reproduces the HCN ground state geometry quite accurately and has been used to tentatively identify HNC in the interstellar medium. The SCF calculations predict HNC to lie 9.5 kcal/mole above HCN, while CI yields 14.6 kcal/mole. Similarly, barrier heights of 40.2 and 34.9 kcal/mole are predicted by SCF and CI. Thus, the SCF approximation is qualitatively reasonable for HNC → HCN. If HNC is designated by a reaction angle of 180° and HCN by 0°, then the saddle point or transition state is predicted to lie at 73.7°, significantly closer to HCN. A reaction path is determined from the SCF potential surface. The surface will be used in RRKM and classical trajectory studies of the dynamics of this reaction. Copyright © 1975 American Institute of Physics.