An approximate approach to calculate the potential acting on an electron in a molecule and construct the molecular face

The molecular face (MF) consists of all the classical turning points of electron movement in a molecule, and it is an iso-PAEM (the potential acting on an electron in a molecule) contour where PAEM (or one-electron energy) equals the negative value of ionization energy (-IE) of the molecule. It is the classical-forbidden region for an electron movement outside the MF, whereas it is the classical-permitted region enclosed by the MF. In this work, we developed an approximate method for quickly constructing the MF. To evaluate this method, we compared the MFs of the original and approximate methods for a series of molecules, such as some homologues, unsaturated hydrocarbons and haloalkanes. Furthermore, we showed the distributions of Fukui descriptors f+(r) and f−(r) on the MFs for haloalkanes, and heterocyclic compounds. Results showed that the approximate calculations are consistent with the exact calculations not only in representing molecular surface but also in predicting the ordering of activities. Armed with the new tool, the MFs of the isolated and stacked bases in DNA double helix were obtained for the first time. This work provides a new representation for indicating shape properties, frontier electron densities and chemical reactivities.