The theoretical analysis of the conformations of chlorocarbonyl isocyanate was reinvestigated. Our results show that ab initio molecular orbital (MO) calculations are reliable in determining the cis-trans energy difference ClC(O)NCO (the trans conformer being the more stable), in agreement with experiment. We thus answer positively the recently posed question, "How reliable are ab initio calculations?" (H.-G. Mack, H. Oberhammer and C.O. Della Vedova, J. Mol. Struct., (Theochem), 200 (1989) 277). The failure of the calculations by Mack et al. reproduce the cis-trans separation gap might be due to the geometrical parameters employed in their work. (In the present work, we employed the MP2/6-31G (d,p) optimized geometries.) We extended the scope of the problem by carrying out similar calculations for HC(O)NCO and FC(O)NCO. The calculated geometries were improved by using a correction procedure in order to predict the corresponding r(g) structures. In the two latter species, the cis conformers are predicted to have slightly higher stabilities than the trans conformers.