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Odd difference between RHF and UHF MULT=1 results

Pedro Silva
pedros@ufp.pt

I have run single-point energy jobs of Cl+ at the  B3LYP/6-311+G(3d,2p) level.
The results are as follows:
UHF MULT=3   E= -459.6897657012          S-SQUARED =    2.003
UHF MULT=1   E= -459.6721793735          S-SQUARED =    1.007
RHF MULT=1   E= -459.6120363451

As expected, the lowest energy corresponds to the triplet state with two singly-occupied p orbitals. The following results are strange: I would only expect differences between the RHF and UHF MULT=1 results to arise when a bond is getting too stretched, not in a single-atom system like this one. I get the same RHF energy when I try to use the $VEC group from the UHF MULT=1 as guess. Applying the Ovchinnikov an Labanowski correction to the spin-contaminated UHF MULT=1 energy still leaves a pure singlet energy far below that computed by RHF.
I'm sorry if this comes across as a very silly question, but does any one know the reason for this differences?

Pedro

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