Statistical properties of two particle systems, in which the interaction potentials include the soft repulsion/attraction within a hard rectangular box, are studied using molecular dynamics simulations. The pore size and the potential dependence of van der Waals instability arising from the packing mechanism are investigated. The van der Waals instability strongly depends both on the soft repulsion and on the position of soft attraction in these model systems. An addition of the soft repulsion to the hard-core system gives rise to the van der Waals instability near the position where two particles tend to face each other on the diagonal line of the rectangular box. For the hard-sphere system with the soft repulsion/attractions, the soft attraction significantly enhances the van der Waals instability, whereas, for the square-well spheres with the soft repulsion, the soft attraction reduces the van der Waals instability.