$
\item $\chi=-\frac{1}{V}\mu_0\frac{\partial^2 E}{\partial B^2}
\Rightarrow$
- only second term contributes to $\chi$!
+ only second term contributes to $\chi$!\\
$\chi=-\frac{1}{V}\mu_0\frac{e^2}{4m}<\phi|(x^2+y^2)|\phi>$
\item $<\phi|x^2|\phi>=<\phi|y^2|\phi>=\frac{1}{3}<\phi|r^2|\phi>$\\
$\Rightarrow \chi=-\frac{1}{V}\mu_0\frac{e^2}{6m}