Theoretical background: The second eigenvalue

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.