.. _unitcell:
Unit cell
=========

The group contains the definition of the lattice basis vectors and the position
of atoms within the unit cell. By default the atomic positions are in fractional
coordinates


.. describe:: parameter

    Lattice parameter in meters.

.. describe:: basis

    Matrix of lattice vectors in the format:

.. code-block:: none

    basis = (         
     [a1, b1, c1],
     [a2, b2, c2],
     [a3, b3, c3]);

.. note::
    The basis vectors form the **columns** of this matrix, not the rows.

.. describe:: positions

    Position of atoms in the unit cell (fractional coordinates by default).

.. describe:: coordinate_format = "fractional"

    "fractional" or "cartesian"

    Coordinate format for the :code:`positions` setting.

Example
#######

.. code-block:: none

    unitcell: {
        parameter = 0.3e-9;

        basis = (
         [1.0, 0.0, 0.0],
         [0.0, 1.0, 0.0],
         [0.0, 0.0, 1.0]);

       positions = (
        ("A", [0.00, 0.00, 0.00]));
    };