Similarities between the effects of configurational changes and applied electric fields on the shape of electron densities

Abstract

A dynamical description of molecular shape can be formulated by analysing the response of the electronic density function (or a continuum of electronic isodensity contours) to a change in the variables defining the physical system. The electronic density function of an isolated molecule depends on a number of parameters contained in the molecular hamiltonian. Those variables specifying the nuclear positions are of particular importance because the shape of the electron density depends strongly on them. A perturbation of the hamiltonian, such as an external field, introduces additional variables that can alter the molecular shape. The information provided by the response of the electronic density to all these distortions is valuable in reaching an understanding of chemical reactivity and chemical interactions. In this work, we discuss a new approach, comparing the effects of configurational changes and external fields on the electronic density. We show that, within a restricted framework, there are approximate scaling relationships that allow one to mimic the effect of a bond stretching (a change in nuclear configuration) on the shape of electronic isodensity contours by means of an electric field applied to an equilibrium configuration. In doing this, note that only the most characteristic and simplest case is discussed in detail, and only simple examples on diatomics are provided. However, many of the conclusions should prove to be valid in the case of more complex systems exhibiting preferential directions, such as adsorbed molecules or molecules enclosed in cavities.