The structure and stability of iron–rare gas (argon and xenon) clusters have been theoretically investigated by means of quantum chemical calculations. Magic numbers in the mass spectra of iron–rare gas clusters were observed in previous laser ablation and multiphoton ionization (MPI) experiments. The density functional method is used to confirm the stability and predict the geometry of these clusters. The interaction potentials for Li(4p)-Ar are calculated using the MCSCF-CI method to explain the experimental laser absorption profiles.

In the present work we have investigated the stability of the Fe+-rare gas clusters and correlate the stability to the observed magic numbers using density functional method. We are interested in the most stable geometry of the clusters. Since rare gases are closed shell systems, the electronic configuration of Fe + plays an important role in the binding and shape of these clusters. In the case of Fe+ Ar clusters for n = 1 - 3, the electronic configuration of Fe+ is 3d n 4s1. At n = 4 a spin flip is observed, i.e. the multiplicity changes from sextet to quartet. Thus, from n = 4 onwards, the electronic configuration of Fe+ interacting with argon atoms is 3dn+14s0. We observed higher bond strengths in iron-argon clusters for n = 4 -7 compared to that of n = 1 - 3. Similarly, for the Fe+Xen clusters, where the Fe+ configuration is 3dn+1 for all clusters studied, we observe higher bond energies compared to that of Fe+Arn clusters. Thus, our observation confirms the previous results: that molecular states, which correlate with the 3d n+1 atomic asymptote of the transition metal ion are strongly bound compared to the states derived from 4s13dn configurations. It is known that the binding in transition metal ion rare gas clusters is predominantly electrostatic with the charge induced dipole interaction term as the most important term. However, other terms like charge-induced quadrupole and van der Waals forces may contribute to the bond strengths of iron rare gas complexes. Induced dipole interaction potential is directly proportional to the polarizability of the rare gas atom, and with more polarizable rare gas atom an increase in bond strength is expected. Our calculations agree with the experimental observation for Fe+Xen clusters. We verified that Fe+Xe4 and Fe+Xe6 are the most stable structures and they correspond to the observed magic numbers. In case of Fe+Arn, our results are inconclusive due to the unusually large BSSE correction of Fe+Ar7, which has large atomization energy. However, the geometry of the Fe+ Ar6 is in agreement with the postulates derived from experimental observations. The excessive computational time has prevented us from using larger bases and studying larger clusters. We hope this work will stimulate interest in more theoretical work in an effort to develop faster and efficient algorithms to perform ab intito structure calculations of larger clusters.