Integrated spatial electron populations

The valence MO energies and Mulliken populations for H3PCH2 at various theory levels are shown in Table 6. The low HOMO energy is consistent with the first ionization potentials determined for substituted ylides which are in the range 6-7 eV (Section II.B.l.f). The Mulliken populations show that there is a build-up of electron density on carbon, also reflected in the integrated spatial electron population (ISEP) value of 9.2 at the 3-21G level 65, with a corresponding decrease in electron density of phosphorus. The semiquantitative approach of Jardine et a/.190 gives a similar result. 

J. B. Collins and A. Streitwieser Jr., J. Comput. Chem., 1, 81 (1980). Integrated Spatial Electron Populations in Molecules Applications to Simple Molecules. 

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