Perform standard ground state calculation (with plane wave basis)
c = bulk('C', 'diamond', 3.57)
calc = GPAW(mode=PW(500),
    xc='PBE',
kpts=(8, 8, 8),
   random=True, # random guess (needed if many empty bands required)
occupations=FermiDirac(0.01),
txt='Cd_gs.txt')
c.calc = calc
c.get_potential_energy()
calc.write('Cd_gs.gpw')