  It is commonly believed that organic molecules are physisorbed on the ideal
non-polar surfaces of wide band gap insulators with limited variation of the
electronic properties of the adsorbate molecule. On the basis of first
principles calculations within density functional theory (DFT) and $GW$
approximation, we show that this is not generally true. We find that the
molecular frontier orbitals undergo significant changes when a hydroxy acid
(here we chose gluconic acid) is adsorbed on MgSO$_4$$\cdot$H$_2$O(100) surface
due to the complex interaction between the molecule and the insulating surface.
The predicted trend of the adsorption effect on the energy gap obtained by DFT
is reversed when the surface polarization effect is taken into account via the
many-body corrections.
