  We present a wavefunction-based approach to correlated ab initio calculations
on crystalline insulators of infinite extent. It uses the representation of the
occupied and the unoccupied (virtual) single-particle states of the infinite
solid in terms of Wannier functions. Electron correlation effects are evaluated
by considering virtual excitations from a small region in and around the
reference cell, keeping the electrons of the rest of the infinite crystal
frozen at the Hartree-Fock level. The method is applied to study the ground
state properties of the LiH crystal, and is shown to yield rapidly convergent
results.
