Introduction
1. Electron correlation effects are extremely important to provide an accurate description of the electronic states arising from different occupations of the 3d and 4s orbitals of transition metal atoms.
2 They have calculated the dns2 --> dn+1s1 excitation energies of atoms Sc-Cu using both MP4 and quadratic configuration interaction (QCI) method. While the MP4 was clearly inadequate, they obtained excellent agreement with experiment using QCI method with a mean deviation of 0.14eV after including the effects of relativistic corrections. The remaining deviation was found to be systematic and can be ascribed partly to basis set deficiencies.
3. In this paper, they consider low-lying ionization energies of all the first-row transition metal atoms Sc-Zn using similar theoretical methods.
Theoretical Methods
4. Methods: QCISD, QCISD(T), MP4
5. Basis set: spd, spdf
6. Relativistic corrections
Relativistic corrections are known to be quite important for the transition metal atoms, the differential effects in some cases being as large as 0.5eV.
7. Martin and Hay have calculated such corrections for all the ionic states considered in this study at the HF level. These calculated corrections assume that the inclusion of electron correlation effects does not change the relativistic corrections. However, in the paper, the author was comparing the neutral atoms with the corresponding ions whose behavior may be somewhat different, the effect of electron correlation on the calculated relativistic corrections was not clear. Thus, the author of this paper have used the computed corrections of Martin and Hay.
Results
8. When they using MPn series method, the results is clearly inadequate, especially for Co-Zn. Closer inspection reveals that the large deviations for Co-Zn arise in cases where the d electron population changes from that of the neutral atom, clearly related to the correlation associated with the breaking of a d electron pair.
9.
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