Handbook of Solvents - George Wypych - ChemTech - Ventech!

434 Jacopo Tomasi, Benedetta Mennucci, Chiara Cappelli
Figure 8.4. Location of the minimum energy for H 2O�HF dimer according to the various basis sets and the various
methods. Crosses (×) refer to SCF calculations without CP corrections, full circles (�) to CP corrected calculations.
The ovoidal area refer to an estimate of the location of the minimum at the HF limit (including an estimate of
the error). Left side: minimal basis sets; right side: double valence-shell basis sets.
ΔE CP is by far more corresponding to the exact potential energy functions in calculations
performed at the HF level with a basis set of a small-medium size. Passing to calculations
with larger basis sets, the BSS error is obviously smaller, but the CP correction is
always beneficial.
We give in Figure 8.4 a graphical view of how CP corrections modify the equilibrium
position of the dimer and, at the same time, its stabilization energy.
The same holds for calculations performed at higher levels of the QM theory, beyond
the HF approximation, with decreasing effects of the CP corrections, however. We may
leave this last subject to the attention of the specialists because for the determination of interaction
potential for liquids, the HF approximation is in general sufficient; in some cases it
may be supplemented by applying simple levels of description for electron correlation, as
we have already said.
The CP corrected interaction energy may be decomposed into terms each having a definite
physical meaning, in analogy with what we have exposed in the preceding subsection
for ΔE without CP corrections. There are slightly different ways of doing it. We summarize
here the strategy that more closely follows the physics of the problem. 10 When this correction
is applied, the various terms better satisfy chemical intuition in passing from one dimer
to another of different chemical composition. Each term of the ΔE decomposition is corrected
with an additive term Δ X (X stays for one of the components of the interaction energy)
which is expressed as a difference in the monomers’ energies computed with the opportune
basis set.
Let us consider again the decomposition we have done in eq. [8.19].