### H2 Molecule Energy and Forces Calculation with GPAW (Python)

Source: https://gpaw.readthedocs.io/en/latest/index

This snippet demonstrates how to calculate the potential energy and forces for an H2 molecule using the GPAW code. It utilizes the ASE library for atomic structure manipulation and GPAW's Plane-Wave (PW) mode for calculations. The output includes the calculated energy in eV and forces in eV/Å.

```python
>>> # H2-molecule example:
>>> import numpy as np
>>> from ase import Atoms
>>> from gpaw import GPAW, PW
>>> h2 = Atoms('H2', [(0, 0, 0), (0, 0, 0.74)])
>>> h2.center(vacuum=2.5)
>>> h2.cell
Cell([5.0, 5.0, 5.74])
>>> h2.positions
array([[2.5 , 2.5 , 2.5 ],
       [2.5 , 2.5 , 3.24]])
>>> h2.calc = GPAW(xc='PBE',
...                mode=PW(300),
...                txt='h2.txt')
>>> energy = h2.get_potential_energy()
>>> print(f'Energy: {energy:.3f} eV')
Energy: -6.631 eV
>>> forces = h2.get_forces()
>>> forces.shape
(2, 3)
>>> print(f'Force: {forces[0, 2]:.2f} eV/Å')
Force: -0.64 eV/Å
```

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