A molecular dynamics (MD) formulation of particle swarm optimization (PSO) is established formally. Based on this analogy, the periodic boundary condition (PBC) is introduced to the PSO method by postulating that the swarm environment is similar to crystal bulk materials studied using MD. A combination of soft and hard boundary modes is employed in the formulation of the hybrid version of the proposed PBC. The hybrid version represents a substantial modification of the scenario assumed in theoretical physics but is allowed here to suit the PSO environment. The hybrid PBC proved to be superior to previously established boundary conditions when applied to standard test functions and the practical problem of linear antenna array synthesis.