Hi,
I investigated the stability of the wavefunction of Cl-pyridine in Gaussian 16 at (HF/aug-cc-pvtz stable=opt), but the output stated there is RHF->UHF instability. I was under the impression that the UHF should not be used for closed-shell systems? Am I wrong? Is this some sort of "artifact"? Can anyone provide some guidance?
Thank you very much fo the help.
Hello,
You can use the UHF for the closed-shell molecules, but you should obtain the same wavefunction as in the case of RHF. But, if stability analysis suggests that you have to use UHF instead of RHF, it is worth checking if your molecule is a close-shell singlet. Maybe your molecule's ground state is an open-shell singlet or triplet (both are very unlikely for simple chlorobenzene). Also, your geometry may be far from the minimum; thus, RHF may not be the best choice. You may also check your input. You may have specified the wrong combination of charge and multiplicity.
Thank you for the reply.
The geometry was optimized at MP2/aug-cc-pvtz and converged, the input. The freq calculation also did not find any imaginary frequencies. If I check the HF wavefunction, the stable keyword finds RHF->UHF instability, but if I use B3LYP the wavefunction is stable. If it was an open shell configuration, shouldn't both find the same? The difference of RHF and UHF energy is rather small, approx 2kcal/mol so I was thinking what is wrong here. Maybe I misunderstood how to do the stable calculation. My input is as follows:
%oldchk=previous.chk
%chk=current.chk
# HF/aug-cc-pvtz guess=read geom allcheck stable
Could you maybe give some advice how to approach this problem?
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