def lattice_to_spacegroup_number(lattice):
    """Return the spacegroup number corresponding to the lowest symmetry
    possible for a given Bravais lattice."""
    _lattice_to_spacegroup_number = {'aP': 1, 'mP': 3, 'mC': 5, 'oP': 16,
        'oC': 20, 'oF': 22, 'oI': 23, 'tP': 75, 'tI': 79, 'hP': 143, 'hR': 
        146, 'cP': 195, 'cF': 196, 'cI': 197}
    if not lattice in _lattice_to_spacegroup_number.keys():
        raise RuntimeError('lattice %s unknown' % lattice)
    return _lattice_to_spacegroup_number[lattice]
