  We describe a method for computing near-exact energies for correlated systems
with large Hilbert spaces. The method efficiently identifies the most important
basis states (Slater determinants) and performs a variational calculation in
the subspace spanned by these determinants. A semistochastic approach is then
used to add a perturbative correction to the variational energy to compute the
total energy. The size of the variational space is progressively increased
until the total energy converges to within the desired tolerance. We
demonstrate the power of the method by computing a near-exact potential energy
curve (PEC) for a very challenging molecule -- the chromium dimer.
