calculate total pressure at given volume and temperature

        :param v: unit-cell volume in A^3
        :param temp: temperature in K
        :return: pressure in GPa

def cal_p(self, v, temp):
        """
        calculate total pressure at given volume and temperature

        :param v: unit-cell volume in A^3
        :param temp: temperature in K
        :return: pressure in GPa
        """
        return self.cal_pst(v) + self.cal_pth(v, temp)