  We have developed a practical scheme to construct partly occupied, maximally
localized Wannier functions (WFs) for a wide range of systems. We explain and
demonstrate how the inclusion of selected unoccupied states in the definition
of the WFs can improve both their localization and symmetry properties. A
systematic selection of the relevant unoccupied states is achieved by
minimizing the spread of the resulting WFs. The method is applied to a silicon
cluster, a copper crystal, and a Cu(100) surface with nitrogen adsorbed. In all
cases we demonstrate the existence of a set of WFs with particularly good
localization and symmetry properties, and we show that this set of WFs is
characterized by a maximal average localization.
