PDB (Protein Data Bank)
=======================

The VASP (Vienna Ab initio Simulation Package) is a widely used software package for quantum mechanical molecular dynamics simulations. 
In VASP, the information about the structure, including atomic positions and unit cell parameters, is typically specified in the `POSCAR` file. 
Below is an example of a simplified `POSCAR` file for a water molecule:

.. code-block:: bash

   Water Molecule
   1.0
      5.0  0.0  0.0
      0.0  5.0  0.0
      0.0  0.0  5.0
   O   H
   3   3
   Direct
    0.00000  0.00000  0.00000
    0.75700  0.58600  0.00000
   -0.75700  0.58600  0.00000


In this example:

- The first line indicates a comment.
- The second line is a scaling factor for the lattice vectors (here, set to 1.0).
- The next three lines define the lattice vectors of the unit cell.
- The following line specifies the chemical elements present in the structure (in this case, oxygen (O) and hydrogen (H)).
- The next line indicates the number of atoms of each type.
- The line with "Direct" denotes that the atomic coordinates are given in fractional coordinates.
- The subsequent lines provide the fractional coordinates of each atom.

This `POSCAR` file represents a water molecule with a cubic unit cell. The actual structure and unit cell
 parameters should be adjusted based on the specific system you are studying. 
 Always consult the VASP documentation for accurate and up-to-date information on the `POSCAR` file format and parameters.