We present an efficient Monte-Carlo method for long-range interacting systems to calculate free energy as a function of an order parameter. In this method, a variant of the Wang-Landau method regarding the order parameter is combined with the stochastic cutoff method, which has recently been developed for long-range interacting systems. This method enables us to calculate free energy in long-range interacting systems with reasonable computational time despite the fact that no approximation is involved. This method is applied to a three-dimensional magnetic dipolar system to measure free energy as a function of magnetization. By using the present method, we can calculate free energy for a large system size of 16 3 spins despite the presence of long-range magnetic dipolar interactions. We also discuss the merits and demerits of the present method in comparison with the conventional Wang-Landau method in which free energy is calculated from the joint density of states of energy and order parameter.KEYWORDS: Monte Carlo, long-range interacting system, Wang-Landau method, free energy measurement, magnetic dipolar systemIn general, Monte Carlo (MC) simulations in longrange interacting systems are much more difficult than those in short-range interacting systems because we have to take a large number of interactions into consideration. For example, in the case of systems with pairwise interactions, the number of interactions is proportional to N 2 , where N is the number of elements of the system. Therefore, if a naive MC simulation is carried out in such systems, the computational time per MC step rapidly increases in proportion to N 2 , which is in contrast to that in the case of short-range interacting systems in which the computational time increases in proportion to N . In order to overcome this difficulty, many simulation methods have been proposed.
1-11)Recently, one of the authors and Matsubara have developed an efficient MC method called the stochastic cutoff (SCO) method for long-range interacting systems.
12)In the SCO method, each of the pairwise interactions V ij is stochastically switched to either 0 or a pseudointeractionV ij by the stochastic potential switching (SPS) algorithm. 13,14) The switching probability to 0 and that toV ij are P ij and 1 − P ij , respectively. Since the pseudointeractionV ij and switching probability P ij are chosen properly in the SPS algorithm, the SCO method strictly satisfies the detailed balance condition concerning the original Hamiltonian. [13][14][15] This means that the SCO method does not involve any approximation. Furthermore, since most of the distant and weak interactions are switched to 0 and an efficient method to switch potentials has been developed, 12) the SCO method enables us to reduce the computational time of long-range interactions markedly. For example, in the case of threedimensional dipolar systems, to which our new method will be applied later, the computational time is reduced from O(N 2 ) to O(N log N ) by the SCO method. 12) We can measure internal...