# Specification of the unit cell

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

##### Copyright (C) 2016-2017 ABINIT group (FJ)

Mentioned in topic_SmartSymm, topic_crystal, help_features#2.1.

### Table of content:

- 1. Introduction.
- 2. Related lesson(s) of the tutorial.
- 3. Related input variables.
- 4. Selected input files.

### 1. 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.

More information about the way the real space lattice, the reciprocal lattice, and symmetries are defined in ABINIT can be found here.

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 SmartSymm.
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### 2. Related lesson(s) of the tutorial.

- Fourth basic lesson Determination of the surface energy of aluminum (100): changing the orientation of the unit cell.
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  ### 3. Related input variables.

  Basic input variables:

  ... acell [CELL lattice vector scaling]
  ... rprim [Real space PRIMitive translations]

  Useful input variables:

  ... angdeg [ANGles in DEGrees]
  ... brvltt [BRaVais LaTTice type]
  ... chkprim [CHecK whether the cell is PRIMitive]
  ... scalecart [SCALE CARTesian coordinates]
  ... spgroup [SPace GROUP number]

  Relevant internal variables:

  ... %rprimd [Real space PRIMitive translations, Dimensional]
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  ### 4. Selected input files.

  WARNING : as of ABINITv8.6.x, the list of input files provided in the specific section of the topics Web pages is still to be reviewed/tuned. In some cases, it will be adequate, and in other cases, it might be incomplete, or perhaps even useless.
  The user can find some related example input files in the ABINIT package in the directory /tests, or on the Web:

  tests/v1/Input: t96.in

  tests/v2/Input: t59.in t60.in

  tests/v3/Input: t23.in

  tests/v5/Input: t77.in

  tests/v6/Input: t16.in t30.in

  tests/tutorial/Input: tbase4_4.in
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