minimum energy configuration/binding energy of a molecule in a solution on a surface

Hello,

I am trying to find the min. energy. conf. of molecule A in methanol on silver (111).

Can you please check if my procedure is correct?

1. Define and geometry optimize the molecule, methanol and silver surface (clave and make super cell with silver)
2. make a periodic cell out of methanol and molecule A
3. Make a layer (cyrstal) with silver and methanol+A cell (HERE IT ASKS ABOUT THE VACUUM SLAB. WHY DO I NEED IT? HOW CAN I DETERMINE THE VACUUM SLAB LENGTH?)
4. Geometry optimization
5. Quenching

I use force field compass,
summation method: electrostatic: ewald    van der waals: atom based