calculate thermal pressure along hugoniot

        :param v: unit-cell volume in A^3
        :return: thermal pressure along hugoniot in GPa

def _hugoniot_pth(self, v):
        """
        calculate thermal pressure along hugoniot

        :param v: unit-cell volume in A^3
        :return: thermal pressure along hugoniot in GPa
        """
        temp = self._hugoniot_t(v)
        return self.cal_pth(v, temp)