another concern is that in B3LYP, increasing the %HF, a difference in spin distribution was found... So the problem now is that in this method, we can make some arbitrary increase/decrease to this component which makes it less theoretical....unless of course we resort to some multireference method which also leads to impractical solution since this approach is not available to many....
Dear Francis Kirby Bokingo Burnea,
The reason behind the overstabilization of the High Spin State over the LS state in HF calculations can be found in the following text:
Boyd was the first to point out that the traditional explanation of Hund’s rule by using the HF model is invalid. From a detailed examination of numerous CI calculations of light atoms, ions, and molecules in their lowlying singlet and triplet states, Boyd arrived at the conclusion that the electron-electron repulsion was greater in the high-spin state in every neutral species and that the lower energy of the high-spin state was due to its greater electron-nucleus attraction. That is, the lower energy of the high-spin state was a consequence of the Fermi and Coulomb correlation between electrons with parallel spins, which enable them to feel a less screened, or more attractive, potential from the nucleus. A simultaneous contraction of the electron cloud leads to a lowering of the electron-nucleus potential energy at the cost of increasing the electron-electron interaction energy in the high-spin state.
R. J. Boyd, Nature 310, 480 (1984).
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Hoping this will be helpful,
Rafik
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