FINDSYM Help

Harold T. Stokes, Branton J. Campbell, and Dorian M. Hatch, Department of Physics and Astronomy, Brigham Young University, Provo, Utah, 84602, USA, stokesh@byu.edu

FINDSYM identifies the space group of a crystal, given the positions of the atoms in a unit cell.

Sample input: NaCl

Input:

(1) title line. This line is copied to the output, but is otherwise ignored by the program.

(2) tolerance. This is the accuracy to which dimensions of the unit cell and positions of atoms are known. Units should be the same as those used for the lattice parameters (usually Angstroms). If zero is entered, the default value of 1.0d-6 will be used.

(3a) lattice parameters: a,b,c,alpha,beta,gamma. Give the angles, alpha, beta, gamma, in degrees.

(3b) lattice vectors in cartesian coordinates. If anything is entered in this field, the program will use these lattice vectors instead of the lattice parameters above. If nothing is entered in this field, the program will use the lattice parameters above.

(4) known centering: P (no known centering), I (body-centered), F (face-centered), A,B,C (base centered), R (rhombohedral centered with coordinates of centered points at (2/3,1/3,1/3) and (1/3,2/3,2/3)).

(5) setting in International Tables for Crystallography. (a) monoclinic axes (unique axis in parentheses), (b) cell choice 1, 2, or 3 (for centered monoclinic), (c) orthorhombic axes, (d) origin choice 1 or 2, (e) hexagonal or rhombohedral axes (for trigonal). See Table 4.3.1 in International Tables for Crystallography for more information about monoclinic and orthorhombic axes.

(6) number of atoms. Enter the number of atoms in the known primitive unit cell.

(7) types of atoms. Give the chemical symbol (Na, Cl, etc.) for each atom. The symbol may contain information about net charge (Fe+2,Fe+3,...) or any other characteristic which may distinguish two atomic sites so that a symmetry operation that takes one atom into the other would be forbidden.

You may use multipliers for entering multiple number of atoms of the same type (for example, 2*Na for Na Na).

(8) include magnetic moments. Check if any atoms have magnetic moments.

(9) positions of atoms. Enter the dimensionless coordinates of each atom (fractions of a,b,c). If "include magnetic moments" is checked above, enter the components mx,my,mz, of the magnetic moments for each atom following the coordinates of its position (even if the magnetic moment of that atom is zero).

Output:

(1) space group identified by

(2) CIF file for resulting structure

(3) link to ISOCIF rendition of unit cell

CIF input tags:

In the interactive version, tolerances for lattice-parameter, atomic-position, site-occupancy, and magnetic-moment tolerances can be specified in separate fields. For the command-line version, the tags _iso_acclat, _iso_accpos, _iso_accocc, and _iso_accmag, can be used to specify these values in the input CIF file. In the interactive version, the values of these tags will automatically populate the tolerance fields of the web form when the file is uploaded.

In the interactive version, any term in the "Type of each atom in the unit cell" list that has a negative coefficient will be ignored by the symmetry-detection routine. In the command-line version, the same functionality can be achieved by using the _iso_ignore_atom tag in the main _atom_site loop of the input CIF file, where a "y" or "Y" value means to ignore the atom, and a "." value means to not ignore the atom.

The value of the _iso_expected_sg tag is the space-group number (1-230) or BNS magnetic space-group number of the space group that the user expects FINDSYM to find. The value of the _iso_expected_volrat tag is the user-expected primitive cell-volume ratio (always an integer) of the input-file and output-file structures (input over output). These two tags were implemented for the purpose of preparing a battery of test cases that can be checked whenever the code is updated. When either the space group or primitive-cell-volume ratio found by FINDSYM are different from expected values indicated by these tags, the output file will contain a warning message.