Studies of the importance of shape considering both adsorption molecule and pore geometry were performed by Monte Carlo simulation in grand canonical ensemble (GCMC) for activated carbons (AC). The effects on adsorption capacity and isosteric heat were investigated in different pore sizes using a pseudo-sphere model and a multi-site potential (elongated shape) on triangular-shaped pore. The triangular geometry was considered as a need to allow for the simultaneous interaction of an adsorbed molecule with three graphite walls to account for the high values observed in the isosteric heat of adsorption at low pressures. Kernels of adsorption isotherms were generated by GCMC for different pore sizes considering two potential models for the determination of pore size distribution that allows for the characterization of various micro and mesoporous solids. We propose using a mixed geometry model (slit-triangular) and an elongated molecule potential to characterize activated carbons. The model is used to characterize a family of AC samples both texturally and energetically. The isosteric heat of adsorption was determined from the experimental isotherm by Monte Carlo simulation and the results were contrasted with experimental data obtaining a good agreement. In addition, the work reports the interesting result on the need to use multi-atom potential (together with the mixed model) to predict heat of adsorption values of the order of those reported experimentally.