B3LYP calculations systematic correlation error

Quantum mechanical data can be very efficiently included in parameterizations, because no data conversion is necessary, and properties can be calculated for any point on the PES. However, it is important to realize that the goal of most force fields is to reproduce experiments, not QM results. The chosen QM level puts a limit on the attainable accuracy of the force field. Most systematic errors in the QM method will be reproduced by the force field. In particular for metal systems, it is necessary to use correlated levels, with reasonably flexible basis sets. Some DFT-based methods have proven to give excellent cost/performance ratios. Suitable theoretical levels are discussed more thoroughly by Diedenhofen et al. in Chapter 4. In all the examples given in later sections, the QM data have been obtained at the B3LYP (40) level, using an ECP (41) for the metal and at least valence double- quality basis sets for all atoms. 

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