Hc. Weissker et al., Calculation of optical properties and density of states for systems with huge unit cells - art. no. 035105, PHYS REV B, 6403(3), 2001, pp. 5105
We present a method for the ab initio calculation of spectral properties of
systems with huge unit cells. Translationally invariant systems with eleme
ntary cells containing several hundred atoms in the unit cell are character
ized by a large number of bands in a very small Brillouin zone. This makes
the allocation of energies at different k points to one band impossible. Fo
r that reason a quadratic extrapolation of the band energies is performed s
tarting from a single k point k(o). The band energies and momentum operator
matrix elements are calculated at ko using the projector augmented wave me
thod. The Brillouin-zone integration is carried out by means of the linear
tetrahedron method following a resampling procedure. The viability of the m
ethod is demonstrated for nonprimitive large supercells by reproducing the
absorption coefficient and the density of states of an ideal crystal. The m
ethod is applied to 216- and 512-atom simple-cubic supercells. A germanium
cluster embedded in a host material is treated as an example of a perturbed
system.