Return element for OPLS forcefield atom key.

def opls_notation(atom_key):
    """Return element for OPLS forcefield atom key."""
    # warning for Ne, He, Na types overlap
    conflicts = ['ne', 'he', 'na']
    if atom_key in conflicts:
        raise _AtomKeyConflict((
            "One of the OPLS conflicting "
            "atom_keys has occured '{0}'. "
            "For how to solve this issue see the manual or "
            "MolecularSystem._atom_key_swap() doc string.").format(atom_key))
    for element in opls_atom_keys:
        if atom_key in opls_atom_keys[element]:
            return element
    # In case if atom_key was not found in the OPLS keys dictionary
    raise _AtomKeyError((
        "OPLS atom key {0} was not found in OPLS keys dictionary.").format(
            atom_key))