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