cell keywords

This is a concise list of the common keywords for CASTEP's cell input file. The definitions of the keywords are given in more detail in the section on the cell file.

Keyword in the cell file	Type	Default	Description
`LATTICE_CART` $^1$	B	--	The cell lattice vectors in Cartesian coordinates.
`LATTICE_ABC` $^1$	B	--	The cell lattice vectors specified in $ a, b, c, \alpha, \beta, \gamma $ format.
`POSITIONS_FRAC` $^2$	B	--	The positions of the ions in fraction coordinates with respect to the lattice vectors.
`POSITIONS_ABS` $^2$	B	--	The positions of the ions in absolute coordinates.
`KPOINT_LIST` $^3$	B	--	A list of k-points in the Brillouin zone with associated weights.
`KPOINT_MP_GRID` $^3$	W	--	The k-points defined as a Monkhorst-Pack grid by specifying the grid dimensions in each direction.
`KPOINT_MP_SPACING` $^3$	P	0.1 $Å^{-1}$	The k-points as a Monkhorst-Pack grid by specifying the maximum distance between k-points.
`KPOINT_MP_OFFSET`	V	0,0,0	The offset of the origin of the Monkhorst-Pack grid in fractional coordinates relative to the reciprocal lattice vectors.
`SPECTRAL_KPOINT_PATH` $^4$	B	--	A list of k-points in the Brillouin zone which defines the path along which a band-structure calculation will be performed.
`SPECTRAL_KPOINT_PATH_SPACING`	P	0.1 $Å^{-1}$	Specifies the maximum spacing between k-points along the path for which a band structure calculation will be performed.
`SPECTRAL_KPOINT_LIST` $^4$	B	SCF k-points	A list of k-points at which a band-structure calculation will be performed.
`SPECTRAL_KPOINT_MP_GRID` $^4$	W	--	The k-points for optical matrix element calculations defined as a Monkhorst-Pack grid by specifying the grid dimensions in each direction.
`SPECTRAL_KPOINT_MP_SPACING` $^4$	P	0.1 Å $^{-1}$	The k-points for optical matrix element calculations as a Monkhorst-Pack grid by specifying the maximum distance between k-points.
`SPECTRAL_KPOINT_MP_OFFSET`	V	0,0,0	The offset of the origin of the Monkhorst-Pack grid for optical matrix element calculations in fractional coordinates relative to the reciprocal lattice vectors.
`PHONON_KPOINT_PATH` $^5$	B	--	A list of k-points in the Brillouin zone which defines the path along which a phonon calculation will be performed.
`PHONON_KPOINT_PATH_SPACING`	P	0.1 Å $^{-1}$	Specifies the maximum spacing between k-points along the path for which a phonon calculation will be performed.
`PHONON_KPOINT_LIST` $^5$	B	SCF k-points	A list of k-points at which a phonon calculation will be performed.
`PHONON_GAMMA_DIRECTIONS`	B	See text	The directions in which the gamma point will be approached for calculation of the LO/TO splitting.
`SYMMETRY_GENERATE` $^6$	D	no symmetry	If this is present, the highest symmtery group of the cell will found and the corresponding symmetry operations generated.
`SYMMETRY_OPS` $^6$	B	no symmetry	The symmetry operations that apply to the cell.
`SYMMETRY_TOL`	P	0.01 Å	The tolerance within which symmetry will be enforced.
`IONIC_CONSTRAINTS`	B	no constraints	The constraints on the motion of ions during relaxation or MD.
`FIX_ALL_IONS`	L	`FALSE`	Constrain all ionic positions to remain fixed.
`FIX_ALL_CELL`	L	`FALSE`	Constrain all cell parameters to remain fixed.
`FIX_COM`	L	`TRUE`	Constrain the centre of mass of the ions to remain fixed.
`CELL_CONSTRAINTS`	B	no constraints	The constraints on changes in the cell shape during relaxation or MD.
`SPECIES_MASS`	B	atomic mass	The masses of the ionic species.
`SPECIES_POT`	B	see text	The names of the pseudopotentials associated with each species.
`SPECIES_LCAO_STATES`	B	see text	The number of angular momentum states to use in the LCAO basis set for this species when performing population analysis.
`EXTERNAL_PRESSURE`	B	no pressure	The external pressure tensor.
`IONIC_VELOCITIES`	B	random	The velocities of the ions in Cartesian coordinates.

For the argument types, B indicates block data, P means a physical value, L is a logical value, D is a keyword that may simply be defined (present) or not, V is a real vector and W is an integer vector.

$^1$ Only one of LATTICE_CART and LATTICE_ABC maybe present in a cell file.

$^2$ Only one of POSITIONS_FRAC and POSITIONS_ABS may be present in a cell file.

$^3$ Only one of KPOINTS_LIST, KPOINTS_MP_GRID and KPOINTS_MP_SPACING may be present in a cell file.

$^4$ Only one of SPECTRAL_KPOINT_PATH,SPECTRAL_KPOINTS_MP_GRID, SPECTRAL_KPOINTS_MP_SPACING and SPECTRAL_KPOINT_LIST may be present in a cell file.

$^5$ Only one of PHONON_KPOINT_PATH and PHONON_KPOINT_LIST may be present in a cell file.

$^6$ Only one of SYMMETRY_GENERATE and SYMMETRY_OPS may be present in a cell file.