  Momentum-dependent excitation spectra of the two-dimensional Hubbard model on
the square lattice have been investigated at zero temperature on the basis of
the full self-consistent projection operator method in order to clarify
nonlocal effects of electron correlations on the spectra. It is found that
intersite antiferromagnetic correlations cause shadow bands and enhance the
Mott-Hubbard splittings near the half-filling. Furthermore nonlocal excitations
are shown to move the critical doping concentration $\delta^{\ast}_{h}$, at
which the singular quasiparticle peak is located just on the Fermi level, from
$\delta^{\ast}_{h}=0.153$ (the single-site value) to $\delta^{\ast}_{h}=0.123$.
The latter suggests the occurance of an instability such as the stripe at
$\delta^{\ast}_{h}=1/8$.
