NCI (Non-Covalent Interactions) is a visualization index based on the density and its derivatives. It enables identification of non-covalent interactions. It is based on the peaks that appear in the reduced density gradient (RDG) at low densities.

There is a crucial change in the RDG at the critical points in between molecules due to the annihilation of the density gradient at these points. When we plot the RDG as a function of the density across a molecule, we see that the main difference between the monomer (Figure 1a) and dimer (Figure 1b) cases is the appearance of steep peaks at low density.

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a) Formic acid

b) Formic acid dimer

Figure 1

When we search for the points in 3D space giving rise to these peaks, non covalent regions clearly appear in the (supra)molecular complex (insets in Figure 1a and Figure 1b).

The interactions revealed by NCI correspond to both favorable and unfavorable interactions. In order to differentiate between them, the sign of the second density Hessian eigenvalue times the density is implemented. This value is able to characterize the strength of the interaction by means of the density, and its curvature thanks to the sign of the second eigenvalue. Some examples are collected in Figure 2. It can be seen that the method is applicable to small molecules as well as inorganic complexes.

NCI (Non-Covalent Interactions) is applicable to promolecular densities, enabling the analysis of biomolecules (Figure 3). In this case, only the atomic coordinates are required as input. Both options (SCF and promolecular are implemented in the code).