How polarizabilities and $C_6$ coefficients actually vary with atomic volume

Abstract: In this work we investigate how atomic $C_6$ coefficients and static dipole polarizabilities $\alpha$ scale with effective volume. We show, using confined atoms covering rows 1-5 of the periodic table, that $C_6/C_6^R\approx (V/V^R)^{p_Z}$ and $\alpha/\alpha^R\approx (V/V^R)^{p'_Z}$ (for volume $V=\int dr \frac{4\pi}{3}r^3 n(r)$) where $C_6^R$, $\alpha^R$ and $V^R$ are the reference values and effective volume of the free atom. The scaling exponents $p_Z$ and $p'_Z$ vary substantially as a function of element number $Z=N$, in contrast to the standard "rule of thumb" that $p_Z=2$ and $p'_Z=1$. Remarkably, We find that the polarizability and $C_6$ exponents $p'$ and $p$ are related by $p'\approx p-0.615$ rather than the expected $p'\approx p/2$. Results are largely independent of the form of the confining potential (harmonic, cubic and quartic potentials are considered) and kernel approximation, justifying this analysis.