Electron Correlation, Including Spin Density Description

Electron Correlation, Including Spin Density Description.—In this Appendix we discuss ... 

The interactions present in halogen-bonded systems are in general weak, thus the need to use quantum mechanical methods that include electron correlation for an accurate description. In particular, these interactions are sensitive, not only to the basis set used but also to the level of electron correlation used, the counterpoise correction and the inclusion of spin-orbit effects for bromine and iodine. In contrast with the widely accepted idea that hydrogen bonded systems are reasonably well described with MoUer-Plesset perturbation (MP2) methods, halogen bond interactions are overestimated at this level. Therefore, for an accurate description of these systems a coupled cluster method is advised, preferably with single and double excitations (CCSD) or with perturbative triple excitations (CCSD(T)). This problem pertains also to density functional theory methods (DFT) where some methods lead to better results than others and the choice of the best functional is not straightforward. DFT methods have the advantage over wave function methods that time... 

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