RHF half-electron technique

With all semi-empirical methods, HyperChem can also perform pseudo-RHF calculations for open-shell systems. For a doublet state, all electrons except one are paired. The electron is formally divided into two half electrons with paired spins. Each half elec- 

Iron carries half the charge of a whole electron. The calculation produces a set of molecular orbitals appropriate for this pseudowave function. HyperChem then assigns the unpaired electron its proper spin (alpha), substitutes this electron in the orbital formerly occupied by the half electrons, and calculates energy and other properties. 

You can think of the half-electron technique as a device to get reasonable orbitals to populate later with whole electrons. 

The half-electron method can also apply to triplet states. For this calculation, HyperChem populates selected molecular orbitals with pairs of half electrons. The final energy is computed by assigning the proper spins. 

An advantage of the half-electron technique is its simplicity. HyperChem can carry it out with only minor modifications of the usual calculation. A disadvantage is that forces may not be accurate because of the half electron approximation. 

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