# 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

{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
}