def interaction2mols(interaction):
    """provide 2 molecules from interaction 
    
    Args:
       interaction: e.g. H2-H2
    
    Returns:
       mol1, mol2

    Examples:

    >>> from exojax.spec.defcia import interaction2mols
    >>> print(interaction2mols("H2-CH4 (equilibrium)"))
    >>> ('H2', 'CH4')
    >>> print(interaction2mols("CH4-He"))
    >>> ('CH4', 'He')

    """
    mm=interaction.split(" ")[0]
    mm=mm.split("-")
    return mm[0],mm[1]