On the 3d 64s2 (5D)−3d 74s1 (5F) separation in Fe

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Abstract
Full CI calculations to determine the 5D−5F separation in Fe using [5s4p2d], [5s4p3d], and [5s4p2d1f] basis sets show that the single reference SDCI procedure is accounting for most of the differential correlation energy in the 3d and 4s shells. The SDCI separation using a [7s6p4d4f2g] basis set, correlating only eight electrons and accounting for the dominant relativistic effects is 1.25 eV, compared to the experimental value of 0.875 eV. Adding the 3s and 3p electrons to the SDCI treatment reduces the separation to 1.06 eV. It is concluded that the differential effect of higher excitations becomes more important when the 3s and 3p electrons are correlated.