def convert_units(value, in_units, out_units, standard='codata2014'):
    """Convert a value, from one unit to another."""
    if in_units == out_units:
        return value
    if standard != 'codata2014':
        raise NotImplementedError('{}'.format(standard))
    if in_units == 'hartree' and out_units == 'eV':
        return value * 27.21138602
    elif in_units == 'bohr' and out_units == 'angstrom':
        return value * 0.5291772105638411
    raise NotImplementedError('{} -> {}'.format(in_units, out_units))
