When BZ is present, information on a polymer, layer, or crystal is written to a
file, <file>.brz. This information can be used by the
program BZ to
explore the band structure in the Brillouin zone of the solid.
Do not attempt to add keyword BZ. Use the utility program
MAKPOL with keyword "LET" - that will make a data set
in the correct format.
In order for BZ to work, certain specific conditions must be
satisfied, these are:
- The data set must consist of a set of fundamental unit cells, in a specific
order. For a solid, the order of unit cells is (0,0,0), (1,0,0), (2,0,0),
... (0,1,0), (1,1,0), ... (0,0,1), (1,0,1), ...(0,1,1), (1,1,1), ... (n,l,m).
Within each unit cell, the order of the atoms must be exactly the same.
Because these conditions make writing the data set very difficult, the utility program
MAKPOL should always be used to create the data set.
MAKPOL is normally used for generating data sets for modeling the chemistry of
solids, mainly organic systems, so the default in MAKPOL is for the data set to
consist of entire molecules. For band-structure work, an extra keyword
"LET" must be added; this suppresses the operation that generates entire
molecules, and instead makes a data set suitable for use with BZ.
- When "LET" is present, only the atoms that make up the unit cell should be
present. The unit cell must also be stoichiometric, that is, there must
be an exact integer multiple of the formula present. For example, TiO2
in the rutile structure has a unit cell with one titanium atom at a vertex and
one at the center of the tetragonal unit cell. There must also be
four oxygen atoms, two inside the unit cell and two on the surface of the unit
- Keyword MERS must be present. This is written out when MAKPOL is run,
and it should not be edited or deleted.
- The Large Unit Cell (LUC) used must be sufficiently large that all phenomena
relating to solid-state effects would not change if the LUC was increased in
size. This condition is normally satisfied if the LUC could hold a sphere
of diameter 9 Ångstroms. For narrow band gap solids
(the semiconductors) and true metals, the LUC might need to be larger, for
example big enough to hold a sphere of diameter 10 or 11 Ångstroms.
- The geometry should not be modified after MAKPOL is
run. This means that if geometry optimization is run, the optimized
geometry should then be used in constructing a new data set for MAKPOL.
Two types of file can be generated by BZ: electronic band
structure files and phonon band structure files. If an electronic band
structure is needed, then at least one SCF calculation must be run, and a normal
output must be generated, i.e., an output that includes the line "FINAL HEAT OF
FORMATION" If a phonon band structure is needed, then a
FORCE calculation must be run. This
calculation normally takes a long time to run, so a useful strategy is to write
out the force matrix (using ISOTOPE) as an insurance
in case anything goes wrong. If something does go wrong, then correct the
error and use RESTART to re-run the end of the job.
To summarize, for an electronic band structure
calculation, use keywords BZ MERS=(n1,n2,n3) and possibly
1SCF, together with a data set generated by the program MAKPOL.
For a phonon band structure calculation, use keywords BZ MERS=(n1,n2,n3) FORCE and possibly