I think the question is incredibly broad, even if it were limited to a single compound. For example: any molecule is hard for Hartree-Fock to be described anywhere close to correct, especially considering the properties you name. Obviously you can go post-HF, then only
almost all molecules are hard to treat. And then there is "MP2 is a graveyard for transition metal chemistry". Also
ab initio is only technically challenging for > 1000 atoms, there is not really a theoretical limit. Usually the smaller molecules are harder to describe. But I guess I am only stating the obvious here. —
Martin - マーチン ♦ 18 secs ago