Density Functional Study of Small Aqeous Be2+ Clusters

We investigate structural, vibrational, and energetic properties of [Be(H$_2$O)$_n$]$^{2+}$ clusters with $n$~= 1, 2, 3, 4, and 6 water molecules relevant to Be$^{2+}$ hydration. The Becke exchange gradient--corrected local density approximation is used in the framework of a pseudopotential representation of the core electrons and a plane wave expansion for the valence orbitals. The calculations were performed without imposing periodic boundary conditions. Good agreement with available Hartree--Fock--based results is observed. In addition we compare these data to results obtained by imposing periodic boundary conditions, and find that the latter have only a minor influence on the results. The stability of the clusters at finite temperature was approached using {\em ab initio} molecular dynamics techniques. We find that the $n=6$ cluster dissociates into a stable [Be(H$_2$O)$_4$]$^{2+}$ complex with attached water molecules in the second hydration shell. The Car--Parrinello methodology used in the present study can be applied without modifications and with the same accuracy to study directly Be$^{2+}$ in liquid water.