# hydrogen atom in MOPAC

Hi Netters!
The most trivial problem in calculating atomic structures is H atom. So
the MOPAC 6.00 calculations gave 11.906 eV (MNDO), 11.396 eV (AM1) and
13.073 eV (PM3) for hydrogen atom ionization potential. There is no
difference in usign spin restricted or unrestricted hamiltonian. The
experimental value is 13.595 eV. The differences between the experiment
and calculations (roughly 1.7/2.2/0.5 for MNDO/AM1/PM3) are two or three
orders of magnitude greater than experimental error. If in this case one
have such difference, what about all other systems? Can somebody give
appropriate explanation?
Rudolf
_________
kiralj : at : olimp.irb.hr