We have to proceed with a different set of approximations . The strength of EHM is that it gives a good qualitative picture of the molecular orbitals. Indeed it was on the basis of looking at the molecular orbitals that came from EHM, that Woodwood and Hoffman devised their famous rules. It can be very poor at predicting energy differences between isomers or even correct geometries. For example, EHM gives NH 3 in the planar configuration a lower energy than the correct pyramidal form.
Water can come out to be linear.
Charge differences, particularly between atoms of very different electronegativity can be grossly exaggerated. However it does give some useful results and insights in some cases. For example, it does very well for ethane, which we will look at next. Next You may be interested in a summary by E Lewers that appeared recently on the Computational Chemistry List. It has some wise words to say about this method.