Figure 1.
Illustration of the fragment, cluster and combined cluster/fragment models for computing NMR chemical shieldings. Depicted is a cross section from the optimized acetaminophen form I molecular crystal with the crystallographically unique molecule shown in blue. The fragment model includes pairwise interactions between the central molecule and any other molecule whose atoms lie within the distance R2bd (blue sphere). The cluster model uses a cluster of molecules surrounding the central molecule, as represented by a licorice model. The cluster/fragment model combines the cluster with pairwise interactions involving other molecules in the blue sphere that are not present in the cluster. In all cases, electrostatic embedding is employed and extends well beyond the two-body cutoff region.