def calc_s11_s22_s33(s2211, s1133):
    """Calculates s11, s22, s33 based on s22-s11 and s11-s33 using
    the constraint s11+s22+s33=0
    """
    s22 = 2.0*(s2211)/3.0 + s1133/3.0
    s11 = s22 - s2211
    s33 = s11 - s1133
    return s11, s22, s33