@classmethod
def get_kinetic_energy_cutoff_charge_density(cls, calc, **kwargs):
    """Return kinetic energy cutoff for charge density in eV.

        .. note :: by default returns 4 * ecutwfc, as indicated in
            http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html
        """
    from qe_tools.constants import ry_to_ev
    parameters = calc.inputs.parameters
    try:
        return parameters.get_dict()['SYSTEM']['ecutrho'] * ry_to_ev
    except KeyError:
        pass
    ecutwfc = cls.get_kinetic_energy_cutoff_wavefunctions(calc)
    if ecutwfc is None:
        return None
    return 4 * ecutwfc
