The unrestricted Hartree-Fock Hamiltonian is to be used.

This is the default for odd-electron (radical) systems because geometry optimization using UHF runs faster than when RHF is used.  This is because the open-shell RHF gradient calculation is relatively slow.

If UHF methods are to be used on even electron systems, add keyword UHF.

A limitation of the UHF method is that the resulting wavefunction is not spin quantized, that is, although <Sz> has a  well-defined value, e.g. 0, 1/2, 1, 3/2, etc., <S2> is normally not quantized.  For a system with an <Sz> of 1/2, the <S2> should be 3/4 = 1/2*(1/2+1), but is usually higher.  This is a consequence of spin contamination.  Spin contamination is not present if UHF is not used.