Chalcopyrite (CuFeS2) is an antiferromagnetic semiconductor with unusual magnetic and electrical properties, which are still not clearly understood. Neutron diffraction experiments reveal a phase transition at ∼50 K that has been attributed to an unexpected appearance of magnetic moments on Cu ions, having a paramagnetic arrangement down to 50 K and then ordering to an antiferromagnetic state at lower temperatures. In this study we use DFT-based computational methods to investigate the electronic structure and magnetic properties of CuFeS2 in order to obtain a reliable source of information for the interpretation of the observed magnetic behavior, and in particular to shed some light on the magnetic behavior of copper atoms in this compound. We have calculated the electronic structure of the ground and low-energy magnetically excited states and deduced a set of exchange coupling constants that are used afterward in classical Monte Carlo simulations to obtain magnetic susceptibility data, which compare successfully with our experimental results above ∼170 K. From our results it can be inferred that copper atoms remain in a diamagnetic state in this temperature range, although spin delocalization from neighboring iron atoms results in a non-negligible spin density on the copper atoms at high temperatures.