  The variational cluster approach (VCA) based on the self-energy functional
theory is applied to the two-dimensional symmetric periodic Anderson model at
half filling. We calculate a variety of physical quantities including the
staggered moments and single-particle spectra at zero temperature to show that
the symmetry breaking due to antiferromagnetic ordering occurs in the strong
coupling region, whereas in the weak coupling region, the Kondo insulating
state without symmetry breaking is realized. The critical interaction strength
is estimated. We thus demonstrate that the phase transition due to competition
between antiferromagnetism and Kondo screening in the model can be described
quantitatively by VCA.
