A new method to study intermolecular interactions via direct electron density measurements has been proposed. This approach involves the sum of constituent molecular electron densities as a reference rather than the usual sum of spherical atom densities for the evaluation of deformation densities. One may thus be able to focus specifically on the regions involving dominant intermolecular interactions such as hydrogen bonding, charge transfer, etc.

A new method to extract the Compton profile from the electron density has been proposed. The method is based on the nonlocal density approximation (nlda) due to Gunnarssonet al and Alonso and Girifalco. Initially the reduced first order density matrix has been estimated from the knowledge of the electron density alone through the use of an averaged density distribution,$$\tilde \rho (r)$$. The reduced first order density matrix thus obtained has then been employed to extract the reciprocal form factor,B(r). The Compton profile,J(q) may thereby be obtained by a cosine transformation ofB(r). These results for theJ(q) are in good agreement with their near Hartree-Fock counterparts.

It has been rigorously established by means of classical electrostatic arguments, that molecular electrostatic potential maps are devoid of local maxima. This forms a generalization of the earlier works of Politzer and co-workers which were restricted to the case of atoms.

The momentum space perspective of a typical dissociative reaction viz. that of H₂O along aC_2ν constrained path is brought out. The tool for the investigation of such a reaction path is provided by the distinctive topography of molecular electron momentum densities (EMD's). The EMD as well as Laplacian of EMD atp=0 are seen to follow the reaction path. The most important outcome of this work is that the information aroundp=0 seems to be sufficient for the momentum space follow-up of a chemical reaction. Qualitative features of the EMD topography for this reaction seem to remain unaffected by the choice of basis-set beyond double-zeta quality. The inclusion of correlation has seemingly no major effects on the qualitative nature of molecular EMD topographies.

The topographical analysis of molecular electron density (MED) and molecular electrostatic potential (MESP) offers insights into the bonding and reactivity patterns through the critical points (CPs) of these scalar fields. The MESP is found to be particularly useful for describing sites of electrophilic attack andweak intermolecular interactions. MESP is also shown to clearly distinguish between the lone pairs and π-delocalization. The concept of atoms in molecules (AIM) which has so far been primarily based on the gradients of MED, has recently been extended via the use of MESP. The portrayal of AIM through MESP clearly reveals the electron rich atoms in the molecule and also provides the details of the preferred direction of approach of an electrophile. This perspective briefly summarizes the prominent features of MESP topography and providesa future outlook.

Quinones are known to perform diverse functions in a variety of biological and chemical processes as well as molecular electronics owing to their redox and protonation properties. Electrostatics chiefly governs intermolecular interaction behaviour of quinone states in such processes. The electronic distribution of aprototypical quinone, viz., p-benzoquinone, with its reduction and protonation states (BQS) is explored by molecular electrostatic potential (MESP) mapping using density functional theory. The reorganization of electronic distribution of BQS and their interaction with electrophiles are assessed for understanding themovement of ubiquinone in bacterial photosynthetic reaction centre, by calculating their binding energy with a model electrophile viz., lithium cation (Li ⁺) at B3LYP/6-311+G(d,p) level of theory. The changes in the values of the MESP minima of BQS states alter their interacting behaviour towards Li ⁺ . A good correlation is found between the value of MESP minimum of BQS and the Li ⁺ binding strength at the respective site. To acquire more realistic picture of the proton transfer process to quinone with respect to its reduction state in thephotosynthetic reaction center, interaction of BQS with model protonated motifs of serine, histidine as well as NH ⁺₄ is explored. Further, the electronic conjugation of the reduced states of 9,10-anthraquinone is probed through MESP for understanding the switching nature of their electronic conductivity.

The requirement of huge computational resources makes quantum chemical investigations on large molecules prohibitively difficult. In particular, calculating the vibrational IR/Raman spectra of large molecules employing correlated ab initio theory is a herculean task. The present article brings out the utility of ourmolecular tailoring approach (MTA)-based software for accurate yet economic spectral calculations employing one or more desktop computers. Hartree-Fock and density functional theory-based benchmark calculations on test cases containing over 175 atoms and over 2300 basis functions show excellent agreement with their full calculations (FC) counterparts with large savings in the computer time and memory/hard disk requirements. These savings are even more impressive at MP2 level of theory. Our MTA-based software thus represents an art-of-the-possible for computing vibrational IR/Raman spectra using a handful of desktop machines