4 configurations are metastable at best. Both methanol and ethanol (CH$_{3}$OH-dsc and CH$_{3}$CH$_{2}$OH-dsc) have positive adsorption energies. One can speculate however, that the van der Waals force correction may remarkably modify the adsorption energies 25 thus yielding stable configurations for dissociated configurations of both methanol and ethanol. B. Dielectric function and optical losses Optical losses of a system are proportional to the imaginary part of dielectric susceptibility function, ( Im ( ε )) $^{7,10,26}$. Here we discuss the Im ( ε ) spectra cal- culated according to Eqs. 2 and 3 for 7 monolayer thick Ag (111) slab with and without different adsorbed molecules. We also compare these results with the ex- perimental data for bulk Ag $^{27}$. First, the predicted optical spectra for bare Ag (111) surface are considered. Without quasi-particle correction the predicted Im ( ε ) LDA data calculated for a slab with experimental lattice constant shows a remarkable red shift in comparison with experiment. The LDA method used in this work produces compressed lattice of bulk Ag. Effect of lattice compression on electron energy structure results in blue shift of optical spectra 28 thus improving a comparison with experiment as demonstrated in Fig. 2. Optical excitations from the surface states are respon- sible for the calculated increase of Im ( ε ) of Ag-slab in visible and near infrared optical range with respect to bulk (see Im ( ε ) data between 1 to 2.5 eV in Fig. 2). In order to understand the nature of predicted changes in the spectra of Im ( ε ) the projected density of states (PDOS) spectra are calculated for the systems studied. Details of the calculations of PDOS are described in Ref. 16. Our previous analysis of calculated band struc- ture and PDOS of the Ag-slab (without molecules) 8 in- dicated substantial contributions of surface electron en- ergy states which are in resonance with the bulk electron states and caused redistribution of the resulting density of states. In this paper we present the PDOS spectra analysis and focus on the effects on PDOS caused by the molecular adsorption. Optical transitions between the occupied states and Fermi surface are responsible for the predicted substan- tial increase of the Im ( ε ) spectra of Ag (111) surface as compared with bulk in the near infrared region as demon- strated in Fig. 2. The transitions between the occupied states near − 1 . 6 eV and Fermi level are responsible for the calculated shoulder at 1.55 eV in Im ( ε ) spectra of bare surface, as shown in Fig. 2. Crystal truncation results in a predicted redistribu- tion of charge between 4 d - and 5 s -orbitals causing effec- tive energy shift of the 4 d -orbitals towards lower energies (see Fig. 2). Based on the PDOS data analysis we can state that predicted modifications of the Im ( ε ) spectra of Ag (111) surface with respect to bulk are caused by surface-to-bulk and bulk-to-surface optical transitions. This conclusion is in the alley of previously reported analysis $^{29}$, thus confirming bulk-to-surface nature of rel- evant electron transitions. Optical features predicted on Ag (111) above 3.5 eV are characterized by bulk elec- tron transitions with minor contributions of the surfaces electrons. Molecular adsorption substantially modifies spectra of Im ( ε ). In this work we study the effect of molecular dis- sociation on the Ag (111) surface. At room temperature most solutions contain dissociated molecules which could interact with the surface states $^{10}$. In Fig. 3 we present the FIG. 2. (a) Projected density of states of bulk (solid blue) and 7 monolayer thick slab (dotted black) of silver. The shift in the d -orbital is due to the redistribution of electrons. (b) Cal- culated spectra of immaginary part of dielectric susceptibil- ity function for bulk (solid orange) and 7 monolayer thick Ag (111) slab (dotted blue) in comparison with experimental data measured in Ref. 27 (circles).