In this paper we describe an Accelerator based on FPGA technology and interfaced to an external host computing system through standard bus connections; it is conceived to accelerate double precision floating point operations, present in the energy calculation of Montecarlo (MC) Metropolis particle system simulations. The Accelerator plays the role of coprocessor giving a speed-up factor equal to 4, with respect last generation PCs. The proposed solution is based on COTS components and shows good characteristics of scalability in terms of clock frequency, memory capability and number of computing units. Moreover, the Accelerator can be also conceived as a part of a computing system made up by a cluster of accelerated workstations.