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# 10.5.6 Noncovalent Interactions (NCI) Plots

(December 20, 2021)

Weitao Yang and co-workers 535 Johnson E. R. et al.
J. Am. Chem. Soc.
(2010), 132, pp. 6498.
, 220 Contreras-García J. et al.
J. Chem. Theory Comput.
(2011), 7, pp. 625.
have shown that the reduced density gradient,

 $s(\mathbf{r})=\left(\frac{1}{2(3\pi^{2})^{1/3}}\right)\frac{\|\hat{\bm{\nabla}% }\rho(\mathbf{r})\|}{\rho(\mathbf{r})^{4/3}}$ (10.21)

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.)