Before BZ can be run, a MOPAC calculation must first be run using keyword
MERS in order to generate the data needed by BZ. Most of this data is invisible to the user, but the user must supply to MOPAC one or two data which BZ will use. These data are:
MERS=(n1,n2,n3)
(See Program MAKPOL and MERS) BZ needs to know how the Fock
or phonon matrix is organized, and MERS gives the number of unit cells in each direction. MOPAC uses the keyword MERS as the prompt to write out a file for use by BZ. Within MOPAC, MERS has no other use.
Data sets using MERS should only be constructed using MAKPOL,
and keyword "LET" must be present in the MAKPOL data set, because the sequence of the atoms is defined by the MERS
keyword. An implication of this is that the MERS keyword should not be
changed in the MOPAC data set. If the keyword needs to be changed, it
should be changed in the data set used by program MAKPOL, and MAKPOL
re-run.
In order to generate all points in the Brillouin zone, it is both necessary and
sufficient to have three unit cells in each direction. That
is, for a polymer, all points in the Brillouin zone can be generated by BZ if MERS=(3)
is used; for a layer system, MERS=(3,3) and for a solid, MERS=(3,3,3).
If the unit cell is very small, more unit cells might be needed. The number of unit cells in each direction must be odd; this prevents any unit cell from being exactly half way to the center of the next Large Unit Cell and thus causing an asymmetry in the set of unit cells, there being one cell at (i,j,k) and no corresponding cell at (-i,j,k).
Having said that, more unit cells might be preferred, in order to
ensure that the cluster is large enough to allow MOPAC to use the gamma point as
being representative of the entire band structure. To verify that the
cluster is large enough, increase the cluster size, and re-run the job. If
the heat of formation per unit cell and the charges do not change significantly,
then the smaller cluster was sufficient for MOPAC. Regardless of how many unit
cells are used, if the heat of formation per unit cell and the charges do not
change significantly on changing the number of unit cells, then the band
structure would also not change on changing the number of unit cells.
BCC
(optional)
Like MERS, BCC is not used by MOPAC. BCC (see
Keywords used by MAKPOL) indicates
that every odd unit cell is missing. If the keyword BCC is present, MOPAC informs BZ
via the BRZ file that every odd unit cell is missing.