.  ![\[f0\](a)The $r$ dependence of crystal field parameter $A_0^0$ and the total energy for O addition on the (000) surface of Nd$_0$Fe$_{00}$B for $\theta=00^\circ$. The closed squares and triangles indicate $A_0^0$ and total energies, respectively. (b)$\theta$ dependence of $A_0^0$ for $r=$ 0.0[Å]{}, 0.0[Å]{}, 0.0[Å]{}, and 0.0[Å]{}.](fig/rfig00.pdf){width="0cm"}  Figure \[f0\](a) shows the $r$-dependence of the values of $A_0^0$ and electronic total energy when the O atom approaches the 