def METHOD_NAME(self, pcliq, pcice, si):
    sstar =  self.wrm.saturation(pcliq);
    sstarprime = self.wrm.d_saturation(pcliq);
    tmp = (1.0 - si) * sstar;
    tmpprime = (1.0 - si) * sstarprime
    G = -self.wrm.d_saturation( pcice + self.wrm.capillaryPressure( tmp + si)) \
      * self.wrm.d_capillaryPressure( tmp + si )
    numer = tmpprime * (1 + G)
    denom = - sstar + G * ( 1.0 - sstar)
    return - numer / denom