# UnitCell

This page gives hints on how to specify the unit cell with the ABINIT package.

## Introduction¶

ABINIT needs three dimensioned non-coplanar vectors, forming the unit cell, to set up the real space lattice.

An initial set of three vectors, specified in real space by rprim or as unit vectors with angles angdeg, are dimensioned in a second step using scaling factors as specified by acell or by rescaling their cartesian coordinates, as specified by scalecart. Internally, only the final result, %rprimd matters. The most detailed explanation can be found by looking at rprim.

Note that ABINIT expects the mixed product of the three vectors (R1xR2).R3 to be positive. If it is not the case, exchange two of them with the associated reduced coordinates. More information about the way the real space lattice, the reciprocal lattice, and symmetries are defined in ABINIT can be found here.

Also note that that Abinit space group routines uses by default strict tolerances for the recognition of the symmetry operations. This means that lattice vectors and atomic positions must be give with enough figures so that the code can detect the correct space group. This is especially true in the case of hexagonal or rhombohedral lattices. Remember that one can specify rational numbers with the syntax:

xred 1/3 1/3 1/3


xred 0.33333 0.33333 0.33333


Using acell and angdeg instead of %rprimd may solve possible issues with the space group recognition.

If your input parameters correspond to a high-symmetry structure but the numerical values at hand are noisy, you may want to increase the value of tolsym in the input file
so that Abinit will resymmetrize automatically the input parameters.

Finally, one can use the structure input variable to initialize the crystalline geometry from an external file. For instance, it is possible to read the crystalline structure from an external netcdf file or other formats such as POSCAR without having to specify the value of natom, ntypat, typat and znucl.

Smart symmetriser

ABINIT has also a smart symmetriser capability, when spgroup!=0 and brvltt=-1. In this case, the CONVENTIONAL unit cell must be input through the usual input variables rprim, angdeg, acell and/or scalecart. ABINIT will fold the conventional unit to the primitive cell, and also generate all the nuclei positions from the irreducible ones. See topic_SmartSymm.

basic:

• acell CELL lattice vector scaling
• rprim Real space PRIMitive translations

useful:

internal:

• %rprimd Real space PRIMitive translations, Dimensional

tutorial:

v1:

v2:

v3:

v5:

v6:

v9:

## Tutorials¶

• Fourth basic tutorial Determination of the surface energy of aluminum (100): changing the orientation of the unit cell.