The multiple-scattering or scattered-wave X-a method

Since the MS-SCF-Xa eigenvalue is a partial derviative of the total energy with respect to occupation number. 

The MS-A a method has been very successful for the calculation of ionization energies for both gas-phase molecules and molecular cluster models of solids. DeAlti et al. (1982) have employed both the MS-Aa method and the LCAO-Aa scheme of Sambe and Felton (1975) in the transition-state procedure for calculating ionization potentials for a number of molecules where the Koopmans approximation within Hartree-Fock theory gives the wrong ordering. They found that the LCAO-Aa ionization potentials were more accurate than the Hartree-Fock Koopmans values, while the MS-Aa results typically showed errors of one or two electron volts. Representative results are given for ozone (O3) in Table 3.8. 

Molecular orbital Experimental value Calculated values  

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