B3LYP vs. B3LYP-D3 (effect of dispersion)
The dispersive function’s primary effect is to account for long-range forces in molecular adsorption on catalyst surfaces by the inclusion of van der Waals forces, as evidenced in Figure 4. Using Grimme’s dispersion with B3LYP amounts to an increase in binding energy of about 0.5 eV for several of the intermediates. This not only brings the energy closer to experimental benchmarks but is necessary for the primary reactant in ammonia synthesis, N2, to be adsorbed on the modeled catalyst. Without it, DFT optimizations do not converge and thus cannot be included in a mechanistic study involving N2as a reactant.
Aside from energetic differences and very small decreases in bond length, the dispersion-corrected B3LYP has a very similar electronic structure as B3LYP. Both methods predict the same ground state spin multiplicity and very similar spin density profiles on the (111) and (100) surfaces before and after adsorption of intermediates. This is consistent with Grimme’s claim that the dispersive term does not rely on nor affect electronic structure.51 Select vibrational modes of adsorbed molecules are affected by dispersion, but most are not. Unadsorbed molecular vibrational modes are not affected at all.