Benchmark Results

The binding energies were calculated corresponding to the process:

{L₁+. . .+L_n}^c−2+ [M(H₂O)_n]²⁺ −→[ML_n]^c+nH₂O (5.1) where the{L_i}^c−2 notation is used to denote the overall charge of the (non-interacting) ligands. In almost all cases the number of ligated water molecules before the reaction was equal to the total number of ligands bound to the metal center after the reaction.

It should be pointed out that in the case of the SP complex the starting complex was [M^II(H₂O)₆]²⁺ in octahedral coordination geometry and the SP complex was in a square pyramidal conformation with one water molecule in the second shell. One can define the binding energy∆Ebind as:

∆Ebind(L_n) =E([ML_n]^c) +nE(H₂O)−E([M(H₂O)_n]²⁺)−(∑

i

E(L_i))^c−2 (5.2) Single point calculations were carried out on all reactant and product states using different levels of theory. Both RCCSD(T) and UCCSD(T) were used, but the results were similar so we will only show UCCSD(T) results, which simplifies the comparison to the results featured in Reference [145]. Since we could not obtain the UCCSD(T) results for the SP system we carried out LUCCSD(T0) calculations and evaluated the accuracy of this method. Furthermore, the LUCCSD(T0) method was used as a reference to evaluate the hybrid LMOMO method. In the LMOMO calculations only orbitals containing the metal center were treated at the coupled cluster level, and the rest of

the system at the LRMP2 level. In Figure 5.2 on the example of the copper complexes the interactions for all four systems are shown, including the representation of the used regions. The high level region is represented in red.

Figure 5.2: Reaction mechanism of the Cu complex in (A) linear, (B) tetrahedral, (C) square planar and (D) square pyramidal coordination geometry with one water molecule in the second coordination shell. Orbitals treated at the high level in LUCCSD(T0):LRMP2 calculations are depicted in red.

In the study of Gutten et al. [145] it was already shown that UCCSD(T) results in combination with the aug-cc-pVTZ basis set are reasonably converged, therefore we did not test the convergence of this method. On the other hand, we wanted to investigate the accuracy of LUCCSD(T0) using the same basis set. In the original study of the LUCCSD(T0) method, it was shown that the error of this method is mostly less than 1 kcal/mol, but can go up to 4 kcal/mol [147, 148]. Since the investigated systems in their study did not contain any metal centers our first step was to evaluate the accuracy of the LUCCSD(T0) method in comparison to UCCSD(T).

In Table 5.1 the results obtained using different wavefunction methods including canon-ical, local and hybrid LMOMO methods are shown. As one can see, the difference be-tween canonical and local MP2 results is in most cases negligible. Only in the case of

systems in square planar coordination geometry the difference is slightly larger, but not more than 1.5 kcal/mol. Therefore, we can conclude that the domain approximation almost does not effect the results. On the other hand, the difference between local and canonical coupled cluster results is larger, although not in excess of 3 kcal/mol. This is in agreement with the results obtained in the original study [147, 148]. This difference can be due to the pair approximation. Since the domain approximation does not produce almost any error, there is no error cancellation in this case. It was shown in the previous work that error cancellation between the domain and pair approximation is important and can lead the local coupled cluster results to be in good agreement with canonical ones. Furthermore, one can notice that the difference is present in the systems with anionic ligands, since in those systems the charge is not as well localized as in systems with uncharged ligands. Therefore, the local correlation approximation should be taken as a good approximation if we take into the account that we are dealing with open shell species which are more demanding for the calculation and the degree of uncertainty is higher for these systems.

Before we look into the LMOMO results, we want to compare the LRMP2 and LUCCSD(T0) results. One can see that in almost all cases the difference between these two methods is less than a few kcal/mol, except for the linear complex of [Cu^II(CH₃S)(H₂O)]⁺, where the difference is formidable, reaching 20 kcal/mol. The same difference was observed also in the case of the canonical calculations. Concerning the performance of the LMOMO method one can see that in all systems investigated in this study the difference between the LMOMO and LUCCSD(T0) results is more or less constant and does not amount to more than 1 kcal/mol. It was already seen in the previous study [145] that in these complexes DFT methods deviate more from the UCCSD(T) results dependent of the charge of the ligand. Therefore, we can conclude that the proposed LMOMO method performs significantly better than the DFT methods independent of the charge of the ligands.

Table 5.1: Interaction Energies of the Studied Metal Ions with Model Binding Sites, ∆Eint (Ln), (for the Reaction (L1 + ... + Ln)^c−2 + [M(H2O)n]²⁺ → [MLn]^c + nH2O), Calculated Using Various ab Initio Methods and [M(H2O)n]²⁺ as References.

coord complex UCCSD(T) LUCCSD(T0) LMOMO LRPM2 RMP2

LI [CuX2]^+(a) -295.4 -296.3 -295.9 -318.3 -318.3

[FeY₄] -367.9 -364.9 -365.2 -363.6 -364.8

[MnY₄] -354.0 -351.3 -351.6 -350.2 -351.5

SP [CuX₆]^(d) - -348.7 -348.6 -344.1

Since we observe that LRMP2 gives reasonable results for the investigated systems we wanted to compare the overall performance of the LRMP2 and LMOMO method. There-fore, we plotted normalized Gaussians in Figure 5.3 for both methods. The center of the Gaussian represents the average difference between LRMP2 or LUCCSD(T0):LRMP2 and LUCCSD(T0). The width represents the root mean square deviation (RMSD). As one can see both methods have an average deviation below 1 kcal/mol. However, the main disadvantage of LRMP2 is that the results can deviate much stronger as it is seen from the width of the Gaussian. This is not the case for the hybrid LMOMO method.

These results show that our hybrid LMOMO method gives results which are in good agreement with the full calculations independent of the investigated system.

- 6 - 4 - 2 0 2 4 6 E [ k c a l / m o l ]

- 0 . 4 0

L U C C S D ( T 0 ) : L R M P 2

- 0 . 0 6 L R M P 2

Figure 5.3: Normalized Gaussians centered at the average difference, the width represents the RMSD.

LUCCSD(T0) was taken as a reference.