# Structural tools

## Relabel the Atoms in the Structure (ATOMLABEL)

The ATOMLABEL keyword can be used to change the atomic labels for the atoms in the current structure:

ATOMLABEL template.s


The template string (template.s) is used to build the new atomic names. The format specifiers for this template are:

• %aid: the index for the atom in the non-equivalent atom list.

• %id: the index for the atom in the non-equivalent atom list, counting only the atoms of the same type.

• %S: the atomic symbol, derived from the current atomic number.

• %s: same as %S, but lowercase.

• %l: the current atom label.

## Powder Diffraction (POWDER)

The keyword POWDER calculates the powder diffraction pattern for the current crystal structure:

POWDER [TH2INI t2i.r] [TH2END t2e.r] [{L|LAMBDA} l.r]
[FPOL fpol.r] [NPTS npts.i] [SIGMA sigma.r]
[ROOT root.s] [HARD|SOFT]


Only the $$2\theta$$ range from t2i.r (default: 5 degrees) to t2e.r (default: 90 degrees) will be plotted. The wavelength of the incident radiation is given by l.r (in angstrom, default: 1.5406 angstrom, corresponding to Cu-Kα). The polarization of the X-ray radiation affects the treatment of the resulting intensities. The default is fpol.r = 0, unpolarized light. For synchrotron radiation, use fpol.r = 0.95. npts.i is the number of points in the generated spectrum (deafult: 10001). Gaussian broadening is used on the observed peaks, with width parameter sigma.r (default: 0.05 degrees).

By default, two files are generated: <root>_xrd.dat, containing the $$2\theta$$ versus intensity data, and <root>_xrd.gnu, the gnuplot script to plot it. The name of these files can be changed using the ROOT keyword. The Miller indices of the peaks are written to the standard output.

The HARD and SOFT keywords control how peaks right outside the plot range are treated. In a HARD powder diffraction pattern, the peaks outside the plot range are not computed, and therefore their tails will not appear on the plot even if the corresponding peak is so close that the contribution to the RDF would be significant. In a SOFT powder diffraction pattern, all peaks are computed and represented, even if the maximum is outside the plot range. Default: SOFT.

## Radial Distribution Function (RDF)

The RDF keyword calculates the radial distribution function (RDF) for the current crystal or molecular structure:

RDF [RINI t2i.r] [REND t2e.r] [SIGMA sigma.r] [NPTS npts.i]
[ROOT root.s] [PAIR is1.s is2.s [PAIR is1.s is2.s ...]]
[HARD|SOFT]


The definition of RDF is similar to the one found in Willighagen et al., Acta Cryst. B 61 (2005) 29, but where the atomic charges are replaced by the square root of the atomic number. Each pair of atoms A and B at a distance $$d_{AB}$$ in the system will create a peak in the RDF with a Gaussian shape: $$I_{AB}(r) = \sqrt{Z_AZ_B}\times \exp\left(-\frac{(r-d_{AB})^2}{2\sigma^2}\right)$$ The RDF is constructed from the sum of all $$I_{AB}(r)$$ functions from all unique atom pairs by doing: $${\rm RDF}(r) = \sum_{A<B} \frac{I_{AB}(r)}{r^2N_{\rm cell}}$$ where $$N_{\rm cell}$$ is the number of atoms in the unit cell.

The RDF is plotted from an initial distance t21.r (default: 0) up to a maximum distance t2e.r (default: 25 bohr) using npts.i points in that interval (default: 10001). Gaussian broadening is used with sigma equal to sigma.r (default: 0.05 bohr). The default units of RINI, REND, and SIGMA are bohr in crystals and angstrom in molecules (this can be changed with the UNITS keyword).

Two files are generated: <root>_rdf.dat, containing the RDF versus distance data, and <root>_rdf.gnu, the gnuplot script to plot it. The name of these files can be changed using the ROOT keyword. If PAIR is given, only the distances between atoms of type is1.s and is2.s will contribute to the RDF. Multiple PAIR keywords can be used. The is1.s and is2.s must be the name of an atomic species in the system.

The HARD and SOFT keywords control how peaks right outside the plot range are treated. In a HARD RDF, the peaks outside the plot range are not computed, and therefore their tails will not appear on the plot even if the corresponding peak is so close that the contribution to the RDF would be significant. In a SOFT RDF, all peaks are computed and represented, even if the maximum is outside the plot range. Default: SOFT.

## Compare Crystal and Molecular Structures (COMPARE)

The COMPARE keywords compares two or more structures:

COMPARE [MOLECULE|CRYSTAL] [SORTED|UNSORTED] [XEND xend.r]
[SIGMA sigma.r] [POWDER|RDF] [REDUCE eps.r]
{.|file1.s} {.|file2.s} [{.|file3.s} ...]


If the structures are crystals, COMPARE finds the measure of similarity (DIFF) based on either their radial distribution functions (RDF keyword) or their powder diffraction patterns (POWDER). If they are molecules, all atoms in all structures under comparison may come in the same order (SORTED) or not (UNSORTED, default). If the atoms are sorted, COMPARE finds the translation and rotation that brings the two molecules into closest agreement and report the root-mean-square (RMS) of the atomic positions. If the atoms are unsorted, compare the molecules using the radial distribution functions. If you want to compare somewhat similar molecules with the atoms in different orders, it is a good idea to sort one of them using the MOLREORDER keyword.

In crystals, the default is to use the powder diffraction patterns. Two crystal structures are exactly equal if DIFF = 0. Maximum dissimilarity occurs when DIFF = 1. The crystal similarity measure is calculated using the cross-correlation functions defined in de Gelder et al., J. Comput. Chem., 22 (2001) 273, using the triangle weight. Powder diffraction patterns are calculated from $$2\theta = 5$$ up to xend.r (default: 50). Radial distribution functions are calculated from zero up to xend.r bohr (default: 25 bohr). SIGMA is the Gaussian broadening parameter for the powder diffraction or RDF peaks. The RDF defaults also apply to comparison of unsorted molecules.

In sorted molecules, the root mean square (RMS) of the atomic positions is reported (the units are angstrom, unless changed with the UNITS keyword). The molecular rotation is calculated using Walker et al.’s quaternion algorithm (Walker et al., CVGIP-Imag. Understan. 54 (1991) 358). For the comparison to work correctly, it is necessary that the two molecules have the same number of atoms and that the atoms are in the same sequence.

Whether the molecular or the crystal comparison is used depends on the file formats passed to COMPARE. If the MOLECULE/CRYSTAL keyword is used then the molecule/crystal comparison code is used.

If more than two structures are used in COMPARE, critic2 will calculate the similarity measure between each pair of structures and present the resulting similarity matrix. If the REDUCE keyword is used a threshold (eps.r) is used to determine whether two structures are equal or not. Critic2 then prints a list of unique structures and repeated structures in the output.

The structures can be given by passing an external file to COMPARE. The syntax is the same as in CRYSTAL and MOLECULE: the file format is identified using the file extension. If a dot (“.”) is used instead of a file name, use the current structure (previously loaded with CRYSTAL/MOLECULE). The COMPARE keyword does not require a previous CRYSTAL or MOLECULE keyword. Hence, valid critic2 inputs would be:

COMPARE bleh1.scf.in bleh2.cif
COMPARE bleh1.xyz bleh2.wfx


provided the files exist.

## Transform the Unit Cell (NEWCELL)

The NEWCELL keyword transforms the unit cell used to describe the current crystal structure to a new cell:

NEWCELL {x1.r y1.r z1.r x2.r y2.r z2.r x3.r y3.r z3.r|n1.i n2.i n3.i} [INV|INVERSE]
[ORIGIN x0.r y0.r z0.r]
NEWCELL [{PRIMSTD|STANDARD|PRIMITIVE} [REFINE]]
NEWCELL [NIGGLI|DELAUNAY]


The new unit cell is given by the vectors (x1.r y1.r z1.r), (x2.r y2.r z2.r), and (x3.r y3.r z3.r) in crystallographic coordinates relative to the old unit cell. The x1, x2, x3 vectors must be pure translations of the old cell; either lattice vectors, centering vectors, or combinations of the two. Alternatively, if three integers are given (n1.i n2.i n3.i), NEWCELL builds a supercell with n1.i cells in the a direction, n2.i cells in the b direction, and n3.i cells in the c direction.

NEWCELL unloads all fields (except the promolecular density) and clears the critical point list. If the INV (or INVERSE) keyword is used, the input vectors correspond to the crystallographic coordinates of the old cell in the new coordinate system. A NEWCELL transformation is the inverse of the same transformation using the INV keyword. Optionally, if an ORIGIN vector is given, (x0.r y0.r z0.r), the cell origin is translated to x0. The units of x0 are crystallographic coordinates of the original cell.

The NEWCELL keyword is useful for building supercells or for performing routine but tedious crystallographic transformations. For instance, given a face-centered cubic lattice and the conventional cubic cell one can find the primitive (rhombohedral) cell by doing:

CRYSTAL LIBRARY mgo
NEWCELL 1/2 1/2 0 1/2 0 1/2 0 1/2 1/2


Likewise, if the current cell is rhombohedral, the same NEWCELL order but including the INVERSE keyword transforms to the cubic. That is:

CRYSTAL LIBRARY mgo
NEWCELL 1/2 1/2 0 1/2 0 1/2 0 1/2 1/2
NEWCELL 1/2 1/2 0 1/2 0 1/2 0 1/2 1/2 INVERSE


gives a unit cell and crystal structure description that is equivalent to the initial one read from the library.

NEWCELL also admits specific keywords that perform common transformations to certain cells of interest. The cell can be transformed to:

• STANDARD: standard (canonical) unit cell.

• PRIMITIVE: standard primitive unit cell. Does not transform the cell if the unit cell is already primitive.

• PRIMSTD: standard primitive unit cell. Does the transformation even if the current unit cell is primitive.

• NIGGLI: Niggli-reduced cell for the current lattice. Use a NEWCELL PRIMITIVE first to get the primitive Niggli cell.

• DELAUNAY: Delaunay-reduced cell for the current lattice. Use a NEWCELL PRIMITIVE first to get the primitive Delaunay cell.

The origin is not translated by any of these keywords. The REFINE keyword can be used in combination with STANDARD, PRIMITIVE, or PRIMSTD. If REFINE is used, then the atomic positions are idealized according to the space group symmetry operations. For instance, an atomic coordinate of 0.333 may become 0.3333333… in a hexagonal space group. These transformations use the spglib library. Please consult the spglib manual for more information.

## Calculate Atomic Environments (ENVIRON)

The ENVIRON keyword prints lists of neighbor atoms:

ENVIRON [DIST dist.r] [POINT x0.r y0.r z0.r|ATOM at.s/iat.i|CELATOM iat.i]
[BY by.s/iby.i] [SHELL|SHELLS]


If POINT is given, print the atomic neighbors around the point with coordinates (x0.r y0.r z0.r) in crystallographic coordinates (crystal) or or molecular Cartesian coordinates (molecule, default units: angstrom). Instead, if ATOM is given, print the neighbors around atom iat.i from the non-equivalent atom list or around every atom with atomic symbol at.s (converted internally to atomic number). If CELATOM is used, then print the environment around atom iat.i from the complete list. If neither POINT nor ATOM nor CELATOM are given, print the environments of all non-equivalent atoms in the unit cell.

By default, the environments extend up to 5 angstrom from the central point. The DIST keyword can be used to change this value (by default, dist.r is in bohr in crystals and angstrom in molecules). The BY keyword allows filtering the neighbor list to print only certain kinds of atoms. If iby.i is given, print only atoms whose non-equivalent ID is the same as iby.i. If by.s is given, print only atoms with the same atomic symbol as by.s (converted internally to atomic number). If SHELLS (or SHELL) is given, group the neighbors in shells by distance (1e-2 atomic distance threshold for atoms in the same shell) and non-equivalent ID.

## Calculate Coordination Polyhedra (POLYHEDRA)

The POLYHEDRA keyword calculates the coordination polyhedra in a periodic solid:

POLYHEDRA atcenter.s atvertex.s [[rmin.r] rmax.r]


The polyhedra are built with atom atcenter.s as the center and atvertex.s as the vertices. These strings can refer to an atomic species, if such species exists in the current structure. If not, then the symbols are converted to atomic numbers, and these are used instead. By default, the distance range to consider the central and vertex atom are coordinated is from zero to the sum of covalent radii times the BONDFACTOR. If a single number (rmax.r) is indicated at the end of the POLYHEDRA command, then the distance range goes from zero to rmax.r. If two numbers are indicated, then the distance range goes from rmax.r to rmin.r.

The output of the POLYHEDRA keyword contains a list of non-equivalent atoms of type atcenter.s that have a coordination polyhedron. The number of vertices, minimum and maximum vertex-center distance, number of faces, and polyhedron volume are printed.

## Effective coordination number (ECON)

The coordination number of an atom is typically defined as the number of neighbors that are closest to that atom. This definition can be unsatisfactory for situations where there is a range of bond lengths around the central atom. To address these complex cases, the calculation of an effective coordination number (ECoN) was introduced by Hoppe in 1979. The ECoN of a given atom is calculated by assigning to each atom around it a weight based on their distance. The original procedure is described in Hoppe, Z. Kristallogr. 150 (1979) 23 and examined in detail in Nespolo, Acta Cryst. B, 72 (2016) 51.

The implementation of ECoN in critic2 is slightly different from those two references in that we do not require the user to define coordination polyhedra for the calculation. Instead, the ECoN in critic2 is calculated as a formula that takes into account the distances from the central atom to all other atoms in the crystal. There are two variants of ECoN: iterative, calculated using a self-consistent procedure, and non-iterative.

For a given atom, the ECoN is defined as: $${\rm ECoN} = \sum_{i} w_{i}$$ where the sum runs over all the atoms in the environment of the chosen central atom (or a subset of atoms belonging to a certain species, see below). The weight of the ith atom ($$w_{i}$$) is defined as: $$w_{i} = \exp\left[1-\left(\frac{d_{i}}{d_{\rm av}}\right)^6\right]$$ with $$d_{i}$$ the distance to atom $$i$$. In the non-iterative variant of ECoN, $$d_{\rm av}$$ is the weighted average distance, defined as: $$d_{\rm av} = \frac{\sum_{i} d_{i}\exp\left[1-\left(\frac{d_{i}}{d_{\rm min}}\right)^6\right]} {\sum_{i}\exp\left[1-\left(\frac{d_{i}}{d_{\rm min}}\right)^6\right]}$$ where $$d_{\rm min}$$ is the shortest distance to the central atom for the considered atomic species. In contrast, the iterative variant of ECoN calculates the weighted average distance by solving the non-linear equation: $$d_{\rm av}^{\rm it} = \frac{\sum_{i} d_{i}\exp\left[1-\left(\frac{d_{i}}{d_{\rm av}^{\rm it}}\right)^6\right]} {\sum_{i}\exp\left[1-\left(\frac{d_{i}}{d_{\rm av}^{\rm it}}\right)^6\right]}$$ which is done using a self-consistent iterative procedure.

A typical output for the ECoN keyword is:

# nid->spc   name(nid)->name(spc)     econ      1econ       nd         1nd
1 -> *          Ti -> *           5.9754     5.9743     3.6805     3.6804
1 -> 1          Ti -> Ti          4.8730     4.7197     5.8587     5.8302
1 -> 2          Ti -> O           5.9754     5.9742     3.6805     3.6804
2 -> *           O -> *           3.2704     3.2170     3.7153     3.7056
2 -> 1           O -> Ti          2.9877     2.9871     3.6805     3.6804
2 -> 2           O -> O           7.9476     6.4604     5.1286     4.9802


where nid represents the non-equivalent atom ID of the central atom for which the ECoN is calculated and the values corresponding to different spc are obtained by considering the distances to atoms belonging to those atomic species only (spc=1, 2, etc.) or to all atoms regardless of species (spc=*). Critic2 reports the iterative ECoN (econ), non-iterative ECoN (1econ), iterative weighted average distance (nd, $$d_{\rm av}^{\rm it}$$), and non-iterative weighted average distance (1nd, $$d_{\rm av}$$).

## Pair and Triplet Coordination Numbers (COORD)

The COORD keyword calculates pair and triplet coordination numbers:

COORD [DIST dist.r] [FAC fac.r]


By default two atoms are coordinated if they are within fac.r times the sum of their covalent radii. By default, fac.r is equal to the BONDFACTOR and the internal list of covalent radii is used (see the RADII keyword). The value of fac.r can be changed with the FAC keyword. The atomic radii for atomic species can be changed with using the RADII keyword. If the DIST keyword is used, all atoms within a distance dist.r are coordinated.

On output, COORD will list the number of coordinated pairs per atom in the unit cell and per atomic species. In addition, it will also list all coordinated triplets X-Y-Z, where Y runs over all atoms in the unit cell and over all atomic species.

## Packing Ratio (PACKING)

The PACKING keyword computes the packing ratio of the crystal.

PACKING {COV|VDW|} [PREC prec.r]


With VDW, use the van der Waals radii. With COV, use the covalent radii. If neither VDW nor COV are used, use half of the nearest neighbor distance (in this case, the spheres would not overlap).

In the VDW and COV cases, the calculation is done using a Monte-Carlo sampling of the unit cell. The PREC keyword allows controlling the precision of this calculation. PREC corresponds to the standard deviation in the van der Waals volume divided by the volume itself. The default prec.r is 0.01. The van der Waals and covalent radii can be changed using the RADII keyword.

## Van der Waals volume (VDW)

The VDW keyword calculates the van der Waals volume of a crystal or molecule.

VDW [PREC prec.r]


The calculation is done using a Monte-Carlo sampling of the unit cell or the molecular volume. The PREC keyword allows controlling the precision of this calculation. PREC corresponds to the standard deviation in the van der Waals volume divided by the volume itself. The default prec.r is 0.01. The van der Waals radii are taken from the internal tables, and they can be changed using the RADII keyword.

## Identify Atoms in the Structure Given Their Coordinates (IDENTIFY)

The IDENTIFY keyword identifies the coordinates in the user input by matching them against the internal list of atoms and critical points:

IDENTIFY [ANG|ANGSTROM|BOHR|AU|CRYST]
x.r y.r z.r [ANG|ANGSTROM|BOHR|AU|CRYST]
...
file.xyz
ENDIDENTIFY/END
IDENTIFY file.xyz


If a coordinate is close (1e-4 bohr) to an atom or CP, the corresponding indices as well as the coordinates are written to the output. The input can come as either the coordinates of the points themselves or a filename pointing to an .xyz file. IDENTIFY can be used in environment mode (IDENTIFY/ENDIDENTIFY, with several lines of input) or as a single command when applying it to a single .xyz file.

The default units are crystallographic coordinates in crystals and molecular Cartesian coordinates in molecules (default unit: angstrom). However, these units can be modified with one of the keywords that follow the IDENTIFY command. For specific points, the unit can be changed by specifying the same keywords after the three coordinates. The units in the .xyz file are angstrom (the xyz file also has to have the usual syntax, with the number of atoms in the first line and the title, or a blank line, in the second line).

In addition to the identity of the points, if any, critic2 also provides the vertices of the cube that encompasses all the points in the list that did match an atom or CP.

## Point-Charge Electrostatic Energy (EWALD)

The EWALD keyword calculates the electrostatic energy of the lattice of point charges using Ewald’s method:

EWALD


The atomic charges are defined using the Q keyword.

## Reorder Atoms in a Molecule or Molecular Crystal (MOLREORDER)

The MOLREORDER keyword reorders the atoms in a molecule or molecular crystal to have the same atomic order as a template molecule. The syntax is:

MOLREORDER template.s target.s [WRITE file.s] [MOVEATOMS]


The template molecule is in file template.s. The target file target.s must contain either a molecule or a molecular crystal. If target.s is a molecule, it must contain the same number and types of atoms as the template. If target.s is a molecular crystal, its asymmetric unit must have an integer number of molecules (Z’, see the WHOLEMOLS option in SYMM/SYM). In addition, all molecular fragments in the crystal must have the same number and atom types as the template.

MOLREORDER works by using the weighted graph matching algorithm proposed in Umeyama, S., IEEE PAMI, 10 (1988) 695-703. If there are significant differences between the structures being compared, MOLREORDER may find the wrong atomic permutation, so it is a good idea to always check that the RMS in the output is small. The RMS is obtained by calculating the rotation matrix that yields the best match, in the least squares sense, between the template and the reordered structure. The MOLREORDER keyword can be used without having any molecular or crystal structure loaded.

If WRITE is given, followed by a file name file.s, the target structure with its atoms in the same order as the template is written to that file. In addition, if MOVEATOMS is used, the atoms in the structure written to file.s are moved to match the structure of the template molecule.

## Calculate Dimensions of Uniform k-Point Grids (KPOINTS)

The KPOINTS keyword:

KPOINTS [rk.r] [RKMAX rkmax.r]


calculates the number of k-points in each direction (nk1, nk2, nk3) of a uniform k-point grid with density given by length parameter $$R_k$$ (rk.r). More k-points are allocated in short directions of the crystal according to VASP’s formula: $$n_i = \text{int}\left[\text{max}\left(1,R_k |{\rm b}_i| + \frac{1}{2}\right)\right]$$ where $$n_i$$ is the number of k-points in direction i and $$|{\rm b}_i|$$ is the length of the ith reciprocal lattice vector.

If the KPOINTS keyword is used without any further input, critic2 lists all the distinct uniform k-point grids for values of rk.r from zero up to a value equal to rkmax.r, which can be modified with the optional RKMAX keyword (default: 100 bohr). If a number rk.r follows KPOINTS, the k-point grid dimensions are calculated only for that value.