def _init_chain_type(chain_type):
    """Validation of chain type on initialization
    
    Parameters
    ----------
    `chain_type` : { "heavy", "h", "light", "kappa", "k", "lambda", "l", "alpha", "a", "beta", "b" }
        word / letter representing the chain_type .
        NOTE: "l" is representing the chain type "lambda", NOT "light".
    
    Returns
    -------
    String
        Returns a string representing the chain type if validation is successfull
    
    Raises
    ------
    ValueError
        If the `chain_type` is not part of valid chain types
    """
    if chain_type:
        valid_chain_types = ['heavy', 'h', 'light', 'l', 'kappa', 'lambda',
            'alpha', 'beta', 'a', 'b', 'k']
        if not chain_type.lower() in valid_chain_types:
            raise ValueError(
                f'Please select a valid chain type! {valid_chain_types}')
        return chain_type.lower()
    else:
        return None
