Several parameters in the COSMO calculation can influence the accuracy of the result of the quantum mechanical calculation. Some of these parameters will be discussed. Note that if one chooses different parameters in the COSMO calculation one may also have to reparametrize the ADF COSMO-RS parameters. A list of some of the ADF COSMO parameters.
The atomic cavity radii and the radius of the probing sphere are the same as in Ref. [2], which describes the COSMO-RS method developed by Klamt et al., which is implemented in ADF. The Becke Perdew functional is relatively good for weakly bound systems, but may not be so good in other cases. The basis set TZP is a compromise basis set. For heavy elements like iodine, a TZ2P basis set is required, including the relativistic scalar ZORA method. The Delley type of cavity construction in ADF can give a large number of COSMO points. The XC functional, basis set, and cavity construction chosen in the ADF COSMO calculation have a similar accuracy as those that were used in Ref. [2]. Note that they are not exactly the same as were used in Ref. [2], since in that paper a different quantum mechanical program was used.
In the parametrization for ADF the same geometry was used for the gas phase and the COSMO calculation, which is different than in Ref. [2]. It depends on the actual solvent if reoptimizing the molecule in the COSMO calculation may give better results. Note that the dielectric medium used in the COSMO model has an infinite dielectric constant in the COSMO-RS model. Thus a geometry optimization of the molecule in the COSMO calculation might be more appropriate for a molecule dissolved in water than for a molecule dissolved in n-hexane.
The fit set in ADF is not always able to describe the Coulomb potential accurately at each of the COSMO surface points. In regular ADF calculations this problem is not apparent since the numerical errors in the integrals computed in the vicinity of the COSMO surface have little impact. However, in COSMO calculations this may have some effect. This is why the option C-Mat exact was selected above, instead of the default C-Mat pot option. Another possibility is to add more fit functions, for example, using the ADDDIFFUSEFITFUNCTIONS key in the input for the adf calculation.
Cavity construction
