It is often challenging to perform NCI analysis on large systems, such as proteins, due to the high computational cost of generating wavefunctions and calculating the grid data for NCI. In such cases, the promolecular approximation version of NCI, referred to as NCI pro , can be used. In this method, the actual electron density of the system is replaced by the promolecular density,  pro , which represents the superposition of the electron densities of the atoms constituting the system in their free state. This is equivalent to a zero-order approximation of the actual electron density, assuming no interaction between atoms that leads to electron density transfer or polarization. Since NCI pro depends only on the elements and atomic coordinates, its computational cost is relatively low, making it applicable to systems containing more than 1000 atoms. While the graphical quality and accuracy of the NCI pro map are somewhat lower than those of the NCI map due to the promolecular approximation, the weak interactions typically have little impact on the electron density. Therefore, if the focus is solely on weak interaction regions, the quality of the NCI pro map is usually acceptable and closely resembles the NCI map. An example of NCI pro analysis is shown in Fig. of Ref. , where the prominent vdW interactions between the two -helices of the protein glycophorin A are clearly displayed in the NCI pro map.