In this short communication, we examine the vector field of the gradient of the electronic potential energy density, \(\nabla\!\left[-v(\mathbf{r})\right]\) , in the free 4,5-dichloro-1,2,3-dithiazolium chloride ion pair, in its crystalline form (Appel’s salt), in the picolinic acid N-oxide molecule, and in the hexagonal polymorph of boron nitride (h-BN). The superposition of this field, \(\nabla\!\left[-v(\mathbf{r})\right]\) , with the vector field of the electron density gradient \(\nabla\rho(\mathbf{r})\) , the total static force field \(\mathcal{F}(\mathbf{r})\) , and the electrostatic force field \(\mathbf{F}_{\text{es}}(\mathbf{r})\) reveals a close correspondence in the spatial partitioning of physical space among the first three. In particular, for a typical polar covalent bond \(\omega\) – \(\alpha\) , where \(\omega\) and \(\alpha\) denote the nuclei of the electron-donor and electron-acceptor atoms, respectively, the atomic-like basin \(\Omega_{W}^{\omega}\) , defined in \(\nabla\!\left[-v(\mathbf{r})\right]\) and associated with the \(\omega\) nucleus, permeates into the neighboring atomic basin \(\Omega_{S}^{\alpha}\) , defined in \(\nabla\rho(\mathbf{r})\) and associated with the \(\alpha\) nucleus, thereby following the force-field pseudoatomic basin \(\Omega_{P}^{\omega}\) , defined in \(\mathcal{F}(\mathbf{r})\) and associated with the \(\omega\) nucleus. As is commonly the case for a polar noncovalent interatomic interaction \(\omega\) ··· \(\alpha\) , the corresponding permeation of \(\Omega_{W}^{\omega}\) lags behind that of \(\Omega_{P}^{\omega}\) or is virtually absent. Thus, at least for the compounds examined herein, the behavior of \(\nabla\!\left[-v(\mathbf{r})\right]\) tends to reproduce that of \(\mathcal{F}(\mathbf{r})\) , which—when analyzed jointly with \(\nabla\rho(\mathbf{r})\) and \(\mathbf{F}_{\text{es}}(\mathbf{r})\) —may serve to discriminate between chemical bonds in which the contribution to the electronic charge transferred interatomically is predominantly covalent or predominantly noncovalent.
