The CRYSTAL ab-initio package conceived in Torino and Daresbury is now widely used (cf the corresponding EEC network 'Development and Application of the Hartree-Fock method in materials sciences') but remains a Hartree-Fock scheme. We want to open the road to the explicit treatment of the electronic correlation for infinite periodic systems.
The first step is the direct determination of Wannier (localized) functions, without passing through the Bloch functions. The method rests on the theory of dressed intermediate Hamiltonians developed in Toulouse; starting from strongly localized guessed MOs, one iteratively realizes Brillouin's theorem without full delocalization. Virtual localized MOs are obtained simultaneously. The second step evaluates the correlation energy from local double excitations (with a physically controlled cut-off) and is not more difficult than a molecular calculation. It may go through dressed CI techniques which are both size-consistent and resisting to strong correlations.