Dear Rafik i do not understand your explanation. You take one monomer and do optimization by DFT method. So i think you get charge density only one molecule ie. monomer. So how you predict the charge density for polymer. i think it need also optimize of the polymer structure. but it is difficult by DFT method due to macromolecule so it need higher basis set.
It turns out that the question was not phrased correctly. Therefore, there is no need to answer a question that deals with different topic. Please see the answer of Michael Päch.