RECON is an algorithm for the rapid reconstruction of molecular charge densities and charge density-based electronic properties of molecules, using atomic charge density fragments precomputed from ab initio wave functions. The method is based on Bader's quantum theory of Atoms in Molecules. A library of atomic charge density fragments has been built in a form that allows for the rapid retrieval of the fragments and molecular assembly. The RECON algorithm reads in a molecular database, determines atom types and environments, assigns the closest match from the library of atom types and combines the densities of the atomic fragments to compute a large set of new and traditional QSAR descriptors. QSPR and QSER indices for large pharmaceutical databases or proteins can be computed within seconds.