Valence shell charge concentration VSCC

The Laplacian thus displays where the electronic charge is locally concentrated or depleted [25, 26]. The topology of the valence shell charge concentration (VSCC), the region of the outer shell of an atom over which V p < 0, is in accordance with the Lewis and valence shell electron pair repulsion model. To each local maximum in the VSCC, a pair of bonded or non-bonded electrons can be assigned (Fig. 2). 

Isovalue surfaces of L(r) = 0 for CO (left) and BH3 (right) showing regions of valence-shell charge concentration (VSCC, inside surfaces) and depletion (VSCD, holes) that define the reactive surface and direction of interaction between a Lewis base and a Lewis acid. Adapted from ref [42]. Copyright 1994 Oxford University Press USA. 

Fig. 9 Representations of minus the Laplacian of the electron dcmsity or the L-fimction in methanal (a) constant value contour plots (solid lines for positive values) in the molecular plane the K shell of the C and O atoms is very close to the respective nuclear positions (b) slice through a 3D envelope of the L-function being zero (c) the critical points (maximum =p rp/e = (3,-3) and saddle point = green = (3,-1)) in the valence shell charge concentration (VSCC) region and (d) L-graph, schematically showing the connectivity (edges) between the maxima (at the vertices)...

The outer quantum shell of an atom is divided into an inner region over which < 0 and an outer one over which > 0. The portion of the shell over which V p < 0, is called the valence-shell charge concentration or VSCC. Within this shell is the sphere over whose surface the valence electronic charge is maximally and uniformly concentrated. 

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