We have studied 175 molecular states of the Fe2 diatomic by constructing full potential energy curves dissociating to the ground Fe(5D) + Fe(5D) and first excited Fe(5D) + Fe(5F) dissociation channels by multireference configuration interaction methods and large basis sets. The ground and the first excited states have been detailed by a multitude of plain and explicitly correlated F12 methods at both the valence and core-valence computational levels. The potential curves of most of the states present strong interactions/avoided crossings that trigger a severe non adiabatic behavior. For reasons of completeness, the ground states of the charged species have also been considered.
Reference 14 was accepted for publication on 17 January 2012 before Ref. 15 was even received by the editorial office of Chem. Phys. Lett. (10 April 2012). Unfortunately, and due to the special nature of the particular issue of Mol. Phys., Ref. 14 was finally published in 2003 while Ref. 15 in 2002 without the authors of either papers knowing of the existence of the others’ contribution.
See, for example, in E. Clementi and A. Veillard, J.Chem. Phys. 44, 3050 (1966) and references therein. We should note that the description of two singly coupled electrons into two different spatial functions is due to W. Heisenberg as applied in the He(1S) case [W. Heisenberg, Z. Phys. 38, 411 (1926)]. The same idea has been worked out formally by W. Goddard III and J. Gerratt.
In Ref. 19, the and 7Δu states do not have the same computational needs with respect to their active space composition.