Reversed electron correlation,

It turns out [20,29], that electron correlation may work in the "opposite direction" in encaged atoms compared to that in the free atoms, thus termed reversed electron correlation. 

The quintessence of the reversed electron correlation effect is illustrated in Figure 20 by nonrelativistic HF and RPAE calculated data of the 4s photoionization cross section of free Ca and encaged Ca, Ca C60 near threshold, both at the frozen-cage approximation level, afs A, [20] and dynamical-cage approximation level, afsA [64]. 

The HF wave funetion eontains equal amounts of ionie and eovalent eontributions (Section 4.3), For covalently bonded systems, like H2O, the HF wave funetion is too ionie, and the effect of electron correlation is to increase the covalent contribution. Since the ionic dissociation limit is higher in energy than the covalent, the effect is that the equiUbrium bond length increases when correlation methods are used. For dative bonds, such as metal-ligand compounds, the situation is reversed. In this case the HF wave function dissociates correctly, and bond lengths are normally too long. Inclusion of... 

Any reaction in solution may be considered to result from both EPD-EPA and ED-EA interactions. The sequence of functions follows the functional principle a chemical interaction is initiated when the reacting entity exhibits one of the four functions, while the other reactant exhibits the reverse correlated function. The reaction is completed when the resulting electronic changes are compensated for by the reverse non-correlated functions. This may be illustrated by the reaction of lithium metal with water 3, 5). 

Recent ab initio calculations have attempted to probe the fundamental source of the reversal of H/D preference in ionic as compared to neutral systems, using water as a test base. A harmonic analysis of the potential energy surface of the water dimer, computed with a 6-31G basis set, indicates that the preference for D in the bridging site can be explained in a manner similar to that described earlier for HF - HF. The frequency of the bending motion of the bridging atom is sensitive to its mass this effect leads to a lower vibrational energy of some 0.2 kcal/mol when the heavier D undergoes this motion. The computations indicated that electron correlation has little effect upon this conclusion, even its quantitative aspects. While the treatment was purely harmonic in nature, other calculations have indicated that anharmonicity effects yield very little distinction between one isotopomer and the next. 

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