  Be$^{2+}$(aq) hydrolysis can to lead to the formation of multi-beryllium
clusters, but the thermodynamics of this process has not been resolved
theoretically. We study the hydration state of an isolated Be$^{2+}$ ion using
both the quasi-chemical theory of solutions and ab initio molecular dynamics.
These studies confirm that Be$^{2+}$(aq) is tetra-hydrated. The quasi-chemical
approach is then applied to then the deprotonation of $Be(H_2O)_4^{2+}}$ to
give $BeOH(H_2O)_3{}^{+}}$. The calculated pK$_a$ of 3.8 is in good agreement
with the experimentally suggested value around 3.5. The calculated energetics
for the formation of BeOHBe$^{3+}$ are then obtained in fair agreement with
experiments.
