Definition of the Bravais lattice
In the XML input file lattices for bulk or film unit cells can be defined in the
cell section. The
type of unit cell is selected by using either the BulkLattice or the FilmLattice XML elements. The
definition of the details of the lattice is in both cases similar. The lattice vectors are given in atomic
units (bohr radii). For the bulk lattice the following examples illustrate the different options to declare
the shape of the unit cell:
<bulkLattice scale="1.00"> <bravaisMatrix> <row-1>.0000000000 5.1673552752 5.1673552752</row-1> <row-2>5.1673552752 .0000000000 5.1673552752</row-2> <row-3>5.1673552752 5.1673552752 .0000000000</row-3> </bravaisMatrix> </bulkLattice>
A simple way to define the lattice is to just provide the lattice vectors in the 3x3 Bravais matrix. The attribute scale of the bulkLattice element is a factor the matrix is multiplied with. It can be used to change the size of the unit cell, e.g., to find the optimized lattice constant from a sequence of DFT calculations.
For film lattices several additional attributes have to be set. The attribute dVac defines the length of the unit cell in z-direction. dTilda defines the length of an artificial, virtual, extended unit cell used to determine the set of LAPW basis functions. This is required as the LAPWs for a given unit cell are adapted to periodic problems. For the direction normal to the film plane this periodicity is not present.
The lattice itself can again be declared by providing the Bravais matrix as it is done in the example. However, in this case the entries of the matrix in the third row and in the third column are ignored.
The vacuum energy parameters are also defined in the filmLattice XML element. These are specified by up to two vacuumEnergyParameters elements: For both vacua energy parameters can be provided for two spins. In nonmagnetic systems only the spinUp entry is considered. The entries define the energy parameters relative to the vacuum potential zero, i.e., the potential infinitely far away from the film.
<filmLattice scale="1.00" dVac="47.66" dTilda="51.37"> <bravaisMatrixFilm> <row-1>6.0157233797 .0000000000 </row-1> <row-2>.0000000000 6.0157233797 </row-2> </bravaisMatrixFilm> <vacuumEnergyParameters vacuum="1" spinUp="-.25" spinDown="-.25"/> </filmLattice>
In film mode it is advisable to use the
bravaisMatrixFilm tag with a 2D Bravais matrix.