  We prove a limiting eigenvalue distribution theorem (LEDT) for suitably
scaled eigenvalue clusters around the discrete negative eigenvalues of the
hydrogen atom Hamiltonian formed by the perturbation by a weak constant
magnetic field. We study the hydrogen atom Zeeman Hamiltonian $H_V(h,B) =
(1/2)( - i h {\mathbf \nabla} - {\mathbf A}(h))^2 - |x|^{-1}$, defined on $L^2
(R^3)$, in a constant magnetic field ${\mathbf B}(h) = {\mathbf \nabla} \times
{\mathbf A}(h)=(0,0,\epsilon(h)B)$ in the weak field limit $\epsilon(h)
\rightarrow 0$ as $h\rightarrow{0}$. We consider the Planck's parameter $h$
taking values along the sequence $h=1/(N+1)$, with $N=0,1,2,\ldots$, and
$N\rightarrow\infty$. We prove a semiclassical $N \rightarrow \infty$ LEDT of
the Szeg\"o-type for the scaled eigenvalue shifts and obtain both ({\bf i}) an
expression involving the regularized classical Kepler orbits with energy
$E=-1/2$ and ({\bf ii}) a weak limit measure that involves the component
$\ell_3$ of the angular momentum vector in the direction of the magnetic field.
This LEDT extends results of Szeg\"o-type for eigenvalue clusters for bounded
perturbations of the hydrogen atom to the Zeeman effect. The new aspect of this
work is that the perturbation involves the unbounded, first-order, partial
differential operator $w(h, B) = \frac{(\epsilon(h)B)^2}{8} (x_1^2 + x_2^2) -
\frac{ \epsilon(h)B}{2} hL_3 ,$ where the operator $hL_3$ is the third
component of the usual angular momentum operator and is the quantization of
$\ell_3$. The unbounded Zeeman perturbation is controlled using localization
properties of both the hydrogen atom coherent states $\Psi_{\alpha,N}$, and
their derivatives $L_3(h)\Psi_{\alpha,N}$, in the large quantum number regime
$N\rightarrow\infty$.
