The experimental charge density in the crystal of disodium pentacyanonitrosoferrate (sodium nitroprusside) dihydrate was analyzed in detail by means of Bader's atoms in molecules theory. It was shown that nitroprusside anion is involved in relatively strong self-interactions through the nitroso group. The obtained results agree well with the spectroscopic data, indicating the tendency of the corresponding moiety to the formation of anion-anion association both in solutions and in the crystalline phase.