A reliable description of dense gas-solid two-phase flows of Geldart A particles in gas-fluidized beds at life-size scale is of great practical importance in process industries. The classical two-fluid model, based on the kinetic theory of granular flows (KTGF), provides a very promising theoretical framework for predicting large-scale gas-solid two-phase flows. However, thus far the two fluid model has not been successful in describing gas-solid flows of Geldart A particles. As the kinetic theory was originally developed for cohesiveless particles, it is essential to check if the theory can still work for Geldart A particles, which are slightly cohesive. In this research, a soft-sphere discrete particle model (DPM) is used to study the detailed particle-particle interactions in periodic boundary domains, where interparticle van der Waals forces are taken into account, with no gas phase present. In our simulations, we (1) compare the results for both the hard-sphere and the soft-sphere discrete particle model for cohesiveless particles, with the theoretical predictions obtained from the kinetic theory of granular flows, and (2) study the effect of the cohesive forces in the soft-sphere model and explore a way to modify the current kinetic theory according to the soft-sphere DPM simulation results. The information obtained from these simulations can be further incorporated into the KTGF based two-fluid model.