====== Periodic-boundary conditions ======

Periodic-boundary conditions are indicated by use of the ''PBC'' command. It can take the following options and directive

  * ''%%LATT_TYPE %%'' The shape of the unit cell. ''CUBIC'': cubic unit cell, ''HEXAG'': hexagonal unit cell, ''PARAP'': a parallelpiped shaped cell
  * ''%%BOXSIZE %%'' For cubic unit cells this specifies the length of the side of the cube in Angstroms.
  * ''%%HEXWIDTH, HEXHEIGHT %%'' For hexagonal unit cells this specifies the width and height of the cell in Angstroms.
  * ''%%VEC %%'' A directive which should be followed by 3 numbers to specify a lattice vector. When specifying lattice vectors like this, there are usually 3 instances of such a directive and the vectors will be checked that they conform to the specified ''LATT_TYPE''. This is the only way to specify lattice vectors for ''LATT_TYPE=PARAP''. The program will not error if less than 3 vectors have been given, as in principle 1 or 2 dimensional periodicity could be desired, although this has not been implemented.

To define parallelpiped lattice vectors, something like the following would be used

<code>
{PBC,LATT\_TYPE='PARAP'
VEC,1.0,0.0,0.0
VEC,0.0,1.5,1.5
VEC,0.0,0.0,1.5
}
</code>