To see how the errors in geometric parameters of HCN translate to GEO, the GEO contours are plotted in the right panel of Figure . This is done by calculating the CCSD(T)(full) potential energy surface around the equilibrium geometry. The molecule is linear and all tested approximations get that right. Thus, in the contour plot we show only errors in the two geometric parameters: the C≡N (x-axis) and C-H (y-axis) bond lengths. The poorest performers are HF, M11-L, M06-HF, whose errors are beyond the shown range. First we note that the SE structure (blue dot) is in an excellent agreement with the reference. The B2PLYP geometry is in a very good agreement with both SE and CCSD(T) structures, and has negligibly small GEO value.