A tank of a compact LINAC (Linear Accelerator), operating at 3 GHz, has been designed. It consists of six accelerating cavities, coupled together via a suitable side coupled cavity structure. An accurate model has been implemented in a home-made computer code to take into account the interaction of the proton beam with the resonant structures. Since a number of geometrical parameters needs to be optimized, a Particle Swarm Optimization approach has been employed to search the global solution. The CST PARTICLE STUDIO® (CST PS) tool has been employed to simulate the performance of the tank with the geometrical parameters optimized by the PSO-based code and a beam energy increase from 7 MeV to 8.21 MeV has been calculated.