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.