In this chapter, we will concentrate on novel information-rich descriptors intended not only for quantitative structure-property relationships (QSPRs), but also for quantitative structure-activity relationships (QSARs). We will concentrate on descriptors that are derived from quantum mechanical calculations and that do not rely heavily on the 2D molecular structure. Many such descriptors are derived from local properties calculated at a molecular surface, so that we can consider two main aspects: the local properties themselves and the techniques used to derive descriptors from them. The goal of just about all QSARs and QSPRs is to describe intermolecular interactions. We, therefore, need local properties that describe the individual components of these interactions. These have been reviewed in detail [1], but will be described here once more in order to set the scene for the individual types of descriptors.Intermolecular interactions are conventionally described in terms of four components [2]:
1) Pauli (electron-electron) repulsion 2) Coulomb (electrostatic) interactions 3) Dispersion (van der Waals) attraction 4) Donor-acceptor (Lewis acid-base) interactions.In conventional classical treatments of molecules (e.g., in force fields and similar treatments for docking, scoring, QSAR, etc.), the Pauli repulsion and the dispersion interaction are combined into a single Lennard-Jones [3] or Buckingham [4] potential, although soft-core variations (i.e., ones with a less steep rise in the Pauli repulsion term) are now becoming popular for free-energy perturbation calculations [5]. Coulomb interactions are generally considered to play a dominant role in determining intermolecular interactions because of their strength in the gas phase and their long range. Their importance decreases in polar solutions, but QSAR and QSPR have Statistical Modelling of Molecular Descriptors in QSAR/QSPR. First Edition. Edited