A simple method for predicting electron density in complex flexible molecules. A tribute to F.L. Hirshfeld

A simple method is introduced to decompose the electronic density of a molecule into the sum of fragments. The method is based on Hirshfeldʹs partitioning scheme. For flexible molecules, one can verify that the density of the fragments around a single bond is invariant to an accuracy better than 0.05 eÅ−3. The method has been checked for alkanes, acetone, urea, water dimers, alanine and glycylglycine. The invariance of the density of fragments on each side of the peptide bond is well fulfilled. Moreover, it turns out that some fragment densities can be transferred from one molecule to a bigger one within the same limit of accuracy. This allows for the possibility of predicting the electron density of a complex molecule that cannot be calculated by quantum mechanical methods. The method is compared with Mullikenʹs partitioning used in similar studies by Mezey. The two methods lead to similar conclusions for various conformations of a given molecule. However, transferability, which is the key for efficient use of such schemes, is significantly better with our approach.