  Ionization process of ferrocene (Fc) to produce ferrocenium cation (Fc+) has
been debated as much as the eclipsed and staggered ferrocene conformers. The
present quantum mechanical study reveals that removal of an electron does not
apparently affect the geometry and symmetry of the cation, as the geometric
changes are < 2% with respect to neutral Fc, but produce the fingerprint
orbital 8a1 of Fc+. The excess orbital energy spectrum (EOES) of the \alpha-
and \beta-electrons reveals that the electrons originated from the transition
metal Fe in both core and valence shells experience significant energy changes
in the cation with respect to the neutral ferrocene counterparts, indicating
that the Fe-electrons correlate stronger than electrons from other atoms such
as carbons in ferrocene. The EOES also exhibits that the orbital energies of
the \alpha-electrons in ferrocenium change more significantly than the
\beta-electrons after one \beta-electron being ionized with respect to
ferrocene. The most significant changes upon ionization are dominated by the
Fe-electrons in non-degenerate signature orbitals. That is, 4a1 which is
dominated by the Fe 3s orbital, 3a2 by the Fe 3pz orbital and the 8a1 orbital
(alpha only) by the Fe 3dz2 (one beta-electron is removed from this orbital).
The \Delta DFT and \Delta SCF calculations yield the IP of 6.90 eV and 6.85 eV,
respectively, in excellent agreement to the recent measurements of 6.9 \pn 0.1
eV, suggesting that significant relaxation energy exists whereas electron
correlation energies are largely cancelled out. A further dual space analysis
(DSA) identifies that the momentum profiles of such the singly occupied
fingerprint 8a1 orbital indeed experiences most significant changes in all
orbitals of Fc+, again indicates that the electron removal of ferrocene
ionization is not from the highest occupied molecular orbital.
