C₆ coefficients and dipole polarizabilities for all atoms and many ions in rows 1-6 of the periodic table

Using time-dependent density functional theory (tdDFT) with exchange kernels we calculate and test imaginary frequency-dependent dipole polarizabilities for all atoms and many ns in rows 1-6 of the periodic table. These are then integrated over frequency to produce $C_6$ coefficients. Results are presented under different models: straight tdDFT calculations using two different kernels, "benchmark" tdDFT calculations corrected by more accurate quantum chemical and experimental data, and "benchmark" tdDFT with frozen orbital anions. Parametrisations are presented for 411+ atoms and ions, allowing results to be easily used by other researchers. A curious relationship, $C_{6,XY}\propto [\alpha_X(0)\alpha_Y(0)]^{0.73}$ is found between $C_6$ coefficients and static polarizabilities $\alpha(0)$. The relationship $C_{6,XY}=2C_{6,X}C_{6,Y}/[\alpha_X/\alpha_YC_{6,Y}+\alpha_Y/\alpha_XC_{6,X}]$ is tested and found to work well ($<5$\% errors) in about 80\% of cases, but can break down badly ($>30$\% errors) in a small fraction of cases.