Returns the elemental parameters related to atom size and
    electronegativity which are used for estimating bond-valence
    parameters (bond length) of pairs of atoms on the basis of data
    provided in 'Atoms Sizes and Bond Lengths in Molecules and Crystals'
    (O'Keeffe & Brese, 1991).

    Args:
        el_symbol (str): element symbol.
    Returns:
        (dict): atom-size ('r') and electronegativity-related ('c')
                parameter.

def get_okeeffe_params(el_symbol):
    """
    Returns the elemental parameters related to atom size and
    electronegativity which are used for estimating bond-valence
    parameters (bond length) of pairs of atoms on the basis of data
    provided in 'Atoms Sizes and Bond Lengths in Molecules and Crystals'
    (O'Keeffe & Brese, 1991).

    Args:
        el_symbol (str): element symbol.
    Returns:
        (dict): atom-size ('r') and electronegativity-related ('c')
                parameter.
    """

    el = Element(el_symbol)
    if el not in list(BV_PARAMS.keys()):
        raise RuntimeError("Could not find O'Keeffe parameters for element"
                           " \"{}\" in \"BV_PARAMS\"dictonary"
                           " provided by pymatgen".format(el_symbol))

    return BV_PARAMS[el]