Full CI calculations to determine the5Dminus;5Fseparation in Fe using lsqb;5s4p2drsqb;, lsqb;5s4p3drsqb;, and lsqb;5s4p2d1frsqb; basis sets show that the single reference SDCI procedure is accounting for most of the differential correlation energy in the 3dand 4sshells. The SDCI separation using a lsqb;7s6p4d4f2grsqb; 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 3sand 3pelectrons 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 3sand 3pelectrons are correlated.
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