I have simulated sorption isotherms of nitrogen in a few polymer models prepared by the usual way of density ramping and several cycles of NPT and NVT dynamics to equilibrate the models to obtain densities that compare reasonably with experimental data. I have noted that particularly with lower capacity polymers the adsorption isotherms seem to be overestimated at the start of the adsorption isotherm, and converging with experimentally obtained isotherms at high pressures (20 atm). To convert the average loadings obtained from Sorption to concentrations I used the Peng-robinson EOS to convert fugacity to pressure and also converted the resulting absolute adsorption to excess adsorption for direct comparison to experimental results.

What would the reason be for this overestimation in adsorption at low to moderate pressures?

Any help will be greatly appreciated.

Thank you