def aa_energies():
    """Function holding free energies of transfer between cyclohexane and water for all natural amino acids.
    H. G. Boman, D. Wade, I. a Boman, B. Wåhlin, R. B. Merrifield, *FEBS Lett*. **1989**, *259*, 103–106.

    :return: dictionary with amino acid letters and corresponding energies.
    """
    energies = {'L': -4.92, 'I': -4.92, 'V': -4.04, 'F': -2.98, 'M': -2.35,
        'W': -2.33, 'A': -1.81, 'C': -1.28, 'G': -0.94, 'Y': 0.14, 'T': 
        2.57, 'S': 3.4, 'H': 4.66, 'Q': 5.54, 'K': 5.55, 'N': 6.64, 'E': 
        6.81, 'D': 8.72, 'R': 14.92, 'P': 0.0}
    return energies
