  We study the possibility of controlling the finite size corrections in exact
diagonalization studies quantitatively. We consider the one- and two
dimensional Hubbard model. We show that the finite-size corrections can be be
reduced systematically by a grand-canonical integration over boundary
conditions. We find, in general, an improvement of one order of magnitude with
respect to studies with periodic boundary conditions only. We present results
for ground-state properties of the 2D Hubbard model and an evaluation of the
specific heat for the 1D and 2D Hubbard model.
