def build_etrans_dct(spc_dct_i):
    """  Build an energy transfer dict from a spc dct
    """

    etrans_dct = {}

    mass = spc_dct_i.get('mass', None)
    if mass is not None:
        etrans_dct['mass'] = mass

    ljpar = spc_dct_i.get('lj', None)
    if ljpar is not None:
        etrans_dct['lj'] = ljpar

    edown = spc_dct_i.get('edown', None)
    if edown is not None:
        etrans_dct['edown'] = edown

    return etrans_dct