ポテンシャル
$$ V(x)=E_0\displaystyle\left[ \left(\frac{x}{x_0}\right)^4- \left(\frac{x}{x_0}\right)^2 \right] $$

力
$$ F_x=-\displaystyle\frac{\partial}{\partial x}V(x) =-\frac{E_0}{x}\displaystyle\left[ 4\left(\frac{x}{x_0}\right)^4- 2\left(\frac{x}{x_0}\right)^2 \right] $$