10.5 Visualizing and Plotting Orbitals, Densities, and Other Volumetric Data 10.5.4 Direct Generation of “Cube” Files 10.5.6 Electron Localization Function

10.5.5 Noncovalent Interactions (NCI) Plots

Weitao Yang and co-workers^{444, 178} have shown that the reduced density gradient,

$s(\mathbf{r})=\left(\frac{1}{2(3\pi^{2})^{1/3}}\right)\frac{\|\hat{\bm{\nabla}% }\rho(\mathbf{r})\|}{\rho(\mathbf{r})^{4/3}}$

(10.17)

provides a convenient indicator of noncovalent interactions, which are characterized by large density gradients in regions of space where the density itself is small, leading to very large values of $s(\mathbf{r})$ . Q-Chem can generate noncovalent interactions (NCI) plots of the function $s(\mathbf{r})$ in three-dimensional space. To generate these, set the PLOT_REDUCED_DENSITY_GRAD $rem variable to TRUE. (See the nci-c8h14.in input example in $QC/samples directory.)

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10.5.5 Noncovalent Interactions (NCI) Plots