Ionic Positions

The ionic positions may be specified in fractional coordinates relative to the lattice vectors of the unit cell, or in absolute coordinates. Only one of POSITIONS_FRAC and POSITIONS_ABS may occur in a cell definition file.

%BLOCK POSITIONS_FRAC

$$\begin{array}{ccccc} CCC_1 \vert I_1 & R_{1i} & R_{1j} & R_{... ...} & R_{2j} & R_{2k} & [{\rm MAGMOM}=mm_2]\\ \vdots \end{array}$$

%ENDBLOCK END POSITIONS_FRAC

The first entry on a line is the symbol or atomic number of the ionic species. The correct symbol will be looked up for the atomic species if the atomic number is specified. A symbol can have a maximum of three characters. The first alphabetical characters identify the element, from which default values for atomic mass etc.

The next three entries on a line in POSITIONS_FRAC are real numbers representing the position of the ion in fractions of the unit cell lattice vectors.

If the optional flag ``MAGMOM'' is present on a line, this sets the spin polarisation ( $\frac{N^\uparrow-N^\downarrow}{N_{tot}}$) of the atom for initialisation of the spin density. If this flag is not present a non-spin polarised state will be assumed.

%BLOCK POSITIONS_ABS
[units]

$$\begin{array}{ccccc} CCC_1 \vert I_1 & R_{1x} & R_{1y} & R_{... ...& R_{2y} & R_{2z} & [{\rm MAGMOM} = mm_2]\\ \vdots \end{array}$$

%ENDBLOCK POSITIONS_ABS

The first entry on a line is the symbol or atomic number of the ionic species, as for POSITIONS_FRAC. The next three entries are real numbers representing the position of the ion in Cartesian coordinates.

[units] specifies the units in which the positions are defined. If not present, the default is Å.

The optional flag MAGMOM is defined above under POSITIONS_FRAC.

For transition state searches, structures for the product and intermediate geometry of the system may be input in blocks POSITIONS_FRAC_PRODUCT and POSITIONS_FRAC_INTERMEDIATE respectively, in fractional coordinates, or POSITIONS_ABS_PRODUCT and
POSITIONS_ABS_INTERMEDIATE respectively, in absolute coordinates. The format of these blocks is the same as POSITIONS_FRAC and POSITIONS_ABS as appropriate. The reactant structure will be taken from the main positions block.