Electron density dictionary (rhoCIF) version 1.0.1
Category ATOM_LOCAL_AXES
Name:'_atom_local_axes_[rho]'
Definition:
This category allows the definition of local axes around each
atom in terms of vectors between neighbouring atoms.
High-resolution X-ray diffraction methods enable the
determination of the electron density distribution in crystal
lattices and molecules, which in turn allows for a
characterization of chemical interactions (Coppens, 1997;
Koritsanszky & Coppens, 2001). This is accomplished by the
construction of a mathematical model of the charge density
in a crystal and then by fitting the parameters of such a
model to the experimental pattern of diffracted X-rays. The
model on which this dictionary is based is the so-called
multipole formalism proposed by Hansen & Coppens (1978). In
this model, the electron density in a crystal is described
by a sum of aspherical "pseudoatoms" where the pseudoatom
density has the form defined in the _atom_rho_multipole_* items.
Each pseudoatom density consists of terms representing the
core density, the spherical part of the valence density and
the deviation of the valence density from sphericity. The
continuous electron density in the crystal is then modelled
as a sum of atom-centred charge distributions. Once the
experimental electron density has been established, the
"atoms in molecules" theory of Bader (1990) provides tools for
the interpretation of the density distribution in terms of its
topological properties.
Ref: Bader, R. F. W. (1990). Atoms in molecules: a quantum
theory. Oxford University Press.
Coppens, P. (1997). X-ray charge densities and chemical
bonding. Oxford University Press.
Hansen, N. K. & Coppens, P. (1978). Acta Cryst. A34,
909-921.
Koritsanszky, T. S. & Coppens, P. (2001). Chem. Rev. 101,
1583-1621.
Example:
Example 1 - This example shows how the local axes can be defined
around each atom in terms of vectors between neighbouring
atoms. If necessary, dummy atoms can be introduced into
the atom_site list for this purpose.
loop_
_atom_local_axes_atom_label
_atom_local_axes_atom0
_atom_local_axes_ax1
_atom_local_axes_atom1
_atom_local_axes_atom2
_atom_local_axes_ax2
Ni2+(1) DUM0 Z Ni2+(1) N(1) X
loop_
_atom_site_label
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
_atom_site_occupancy
DUM0 0.80000 0.80000 0.80000 0.0
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Type: null
Category: category_overview
