from ase import Atoms
from gpaw import GPAW

a = 5.475
b = a / 2
si = Atoms(symbols='Si4',
           positions=[(0, 0, 0),
                      (0, b, b),
                      (b, 0, b),
                      (b, b, 0)],
           cell=(a, a, a),
           pbc=True)
si += si
si.positions[4:] += a / 4

calc = GPAW(mode='fd',
            nbands=16,
            h=0.25,
            txt='si.txt')

si.calc = calc
si.get_potential_energy()
calc.write('si.gpw', mode='all')
