  Within this paper we outline a method able to generate truly minimal basis
sets which describe either a group of bands, a band, or even just the occupied
part of a band accurately. These basis sets are the so-called NMTOs, Muffin Tin
Orbitals of order N. For an isolated set of bands, symmetrical
orthonormalization of the NMTOs yields a set of Wannier functions which are
atom-centered and localized by construction. They are not necessarily maximally
localized, but may be transformed into those Wannier functions. For bands which
overlap others, Wannier-like functions can be generated. It is shown that NMTOs
give a chemical understanding of an extended system. In particular, orbitals
for the pi and sigma bands in an insulator, boron nitride, and a semi-metal,
graphite, will be considered. In addition, we illustrate that it is possible to
obtain Wannier-like functions for only the occupied states in a metallic system
by generating NMTOs for cesium. Finally, we visualize the pressure-induced s to
d transition.
