The NCI method has existed for over a decade, and it has had great success in qualitative studies of non-covalent interactions in a variety of systems. It relies on the detection of low-density and low-reduced density gradient volumes in intermolecular complexes that co-locate at sites of interactions making for an excellent visualization tool.

A quantitative aspect has been theorized as the integral over the detected volume, though the derivation of the equation justifying is still elusive. Consistent correlations between NCI indices and binding energies nonetheless suggest its existence. This work sets out to find the symbolic form of this relationship and evaluate its accuracy in the prediction of interaction energies in small dimeric complexes. To that end, it systematically evaluates the integration of NCI volumes, and their dependence on variable parameters, and hence clarifies the definitions of low densities. The symbolic relationship between the NCI index values and gold standard binding energies has been determined for the NCI atlas' hydrogen bond and dispersion datasets and calculated the errors below 3.0 kJ/mol. The resultant equation was laterally tested on the S66 dataset and found to calculate its interaction energies at chemical accuracy. This work is the first step for the creation of a binding energy predictor from electron density alone across different chemical families, therefore for the creation of a tool capable of predicting and calculating NCIs in a variety of structures.