Cell Definition File

The cell definition file is a free-format keyword driven text file, as described in Section . The file defines the initial cell for a calculation. The cell lattice vectors, ionic positions, sampling k-points in the Brillouin zone, cell symmetry, external pressure, constraints on motion of the ions or cell, the pseudopotentials representing each species and the mass of each species may be defined.

At the very least, the cell lattice vectors and ionic positions must be specified. Reasonable defaults, defined in tables 2.1 to 2.3 will be chosen for other variables not specified.

A list of keywords for the cell file is given in Tables 2.1 to 2.3.

Table 2.1: The keywords which may be specified in the cell definition file. Full details of their definitions may be found in Section 2.1.1. Argument types are represented by, B for block data, P for a physical value, L for a logical value, D for a keyword that may simply be defined (present) or not, V for a real vector and W for an integer vector.

Keyword	Type	Default	Description
LATTICE_CART*	B	-	The cell lattice vectors in Cartesian coordinates.
LATTICE_ABC*	B	-	The cell lattice vectors specified in $a, b, c, \alpha, \beta, \gamma$ format.
POSITIONS_FRAC$\dagger$	B	-	The positions of the ions in fraction coordinates with respect to the lattice vectors.
POSITIONS_ABS$\dagger$	B	-	The positions of the ions in absolute coordinates.
POSITIONS_FRAC_PRODUCT$\dagger$	B	-	The positions of the ions in the product structure for a transition state search, in fraction coordinates with respect to the lattice vectors.
POSITIONS_ABS_PRODUCT$\dagger$	B	-	The positions of the ions in the product structure for a transition state search, in absolute coordinates.
POSITIONS_FRAC_INTERMEDIATE$\dagger$	B	-	The positions of the ions in an estimate of the intermediate structure for a transition state search, in fraction coordinates with respect to the lattice vectors.
POSITIONS_ABS_INTERMEDIATE$\dagger$	B	-	The positions of the ions in an estimate of the intermediate structure for a transition state search, in absolute coordinates.
IONIC_VELOCITIES	B	random	The velocities of the ions in Cartesian coordinates.
KPOINT_LIST$\ddagger$	B	-	A list of k-points in the Brillouin zone with associated weights.
KPOINT_MP_GRID$\ddagger$	W	-	The k-points defined as a Monkhorst-Pack grid by specifying the grid dimensions in each direction.
KPOINT_MP_SPACING$\ddagger$	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.

Table 2.2: The keywords which may be specified in the cell definition file. Full details of their definitions may be found in Section 2.1.1. Argument types are represented by, B for block data, P for a physical value, L for a logical value, D for a keyword that may simply be defined (present) or not, V for a real vector and W for an integer vector.

Keyword	Type	Default	Description
BS_KPOINT_PATH*	B	-	A list of k-points in the Brillouin zone which defines the path along which a band-structure calculation will be performed.
BS_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.
BS_KPOINT_LIST*	B	SCF k-points	A list of k-points at which a band-structure calculation will be performed.
OPTICS_KPOINT_LIST$\ddagger$	B	-	A list of k-points in the Brillouin zone with associated weights for optical matrix element calculations.
OPTICS_KPOINT_MP_GRID$\ddagger$	W	-	The k-points for optical matrix element calculations defined as a Monkhorst-Pack grid by specifying the grid dimensions in each direction.
OPTICS_KPOINT_MP_SPACING$\ddagger$	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.
OPTICS_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$\circ$	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$\circ$	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.

Table 2.3: The keywords which may be specified in the cell definition file. Full details of their definitions may be found in Section 2.1.1. Argument types are represented by, B for block data, P for a physical value, L for a logical value, D for a keyword that may simply be defined (present) or not, V for a real vector and W for an integer vector.

Keyword	Type	Default	Description
SYMMETRY_GENERATE$\$$	D	no symmetry	If this is present, the highest symmtery group of the cell will found and the corresponding symmetry operations generated.
SYMMETRY_OPS$\$$	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.

The definitions of the keywords are given in more detail in the following subsections. For the purposes of the following definitions, all variables represented by $R$ are defined to be real numbers, those represented by $I$ are defined to be integers and those represented by $C$ are characters.