  We study intersite electron correlations in the half-filled Hubbard model on
square lattices with periodic and open boundary conditions by means of a
real-space dual fermion approach. By calculating renormalization factors, we
clarify that nearest-neighbor intersite correlations already significantly
reduce the critical interaction. The Mott transition occurs at U/t~6.4, where
$U$ is the interaction strength and t is the hopping integral. This value is
consistent with the quantum Monte Carlo results. This shows the importance of
short-range intersite correlations, which are taken into account in the
framework of the real-space dual fermion approach.
