The internal energies, including transverse and longitudinal parts, of quantum Heisenberg systems for arbitrary spin S are investigated by the double-time Green's function method. The expressions for ferromagnetic (FM) and antiferromagnetic (AFM) systems are derived when one-component of magnetization is considered with the higher order longitudinal correlation functions being carefully treated. An unexpected result is that around the order-disorder transition points the neighboring spins in a FM (AFM) system are more likely longitudinally antiparallel (parallel) than parallel (antiparallel) to each other for Sr3/2 in spite of the FM (AFM) exchange between the spins. This is attributed to the strong quantum fluctuation of the systems with small S values. We also present the expressions of the internal energies of FM systems when the three-component of magnetizations are considered. 2 RPA in order to achieve satisfactory internal energies [18]. That is to say, higher-order Green's functions have to be solved. However, it is very difficult to do so. There has not been much work [20][21][22][23][24][25][26] attempting to solve the higher-order Green's functions and they were usually limited to the low-dimensional lattices and the lowest spin quantum number S¼1/2. Even for the low-dimensional systems, it was difficult to deal with the cases with higher spin quantum numbers. The only instance of dealing with the higher S values was confined in finite lattice site systems [25]. A remarkable progress was the calculation of the internal energies of ferromagnetic (FM) lattices above the Curie point by means of the higher-order Green's functions [27]. There was one common feature in the work presented in Refs. [20][21][22][23][24][25][26][27]: the higher-order Green's functions were constructed in the cases where the magnetization was zero.To sum up, the evaluation of the internal energy of the Heisenberg systems when the magnetization was not zero by means of the Green's function method has seldom been there to see. We believe that under the RPA, it is possible to obtain as good as possible expressions for the internal energy applicable to any S value for nonzero magnetization.The internal energy of a Heisenberg magnetic system mainly includes two parts, the transverse correlation energy (TCE) and longitudinal correlation energy (LCE), as defined in Eqs. (3) and (4) below. The former is easily calculated by means of the well-known spectral theorem without any further approximation [5,18,28]. Hereafter, when no further approximation is made in giving an expression of the energy, we say the expression is precise. In this sense, the expression of the TCE is precise. The LCE, however, can be dealt with precisely only in the case of S¼1/2 and 1 [18,28,29]. For higher S values, the treatment of this part is troublesome. At first thought, the following approximation can be taken [5]