It is straightforward to deduce that by default UHF (or UKS) singlet calculation produce identical alpha and beta orbitals, because there is no reason to break spin symmetry during the optimization (initially orbitals for both spins are the same, as follows from physical considerations). This was, however, new for me (thanks for explanation to Gaussian support team) when I had become suspicious about too good correspondence between RKS and UKS values, despite that I knew DFT is much less subject to spin contamination than HF.
Then, as it might happen if You are using some automation tool to generate input files, I have unintendedly generated some with RHF/... Guess=Mix. As in population analysis found in the output file there is only data on Alpha orbitals, I conclude that "R" or "U" in front of computation method is preferred over the Guess type by the program when it is determining the type of calculation.
UPDATE: First hyperpolarizability values are slightly changing for water and ozone calculations. RHF Guess=Mix seems to be closer to UHF than to RHF, although only alpha orbitals are shown in population analysis. Nevertheless, this difference is marginal. I will check later for some larger molecule.
Then, as it might happen if You are using some automation tool to generate input files, I have unintendedly generated some with RHF/... Guess=Mix. As in population analysis found in the output file there is only data on Alpha orbitals, I conclude that "R" or "U" in front of computation method is preferred over the Guess type by the program when it is determining the type of calculation.
UPDATE: First hyperpolarizability values are slightly changing for water and ozone calculations. RHF Guess=Mix seems to be closer to UHF than to RHF, although only alpha orbitals are shown in population analysis. Nevertheless, this difference is marginal. I will check later for some larger molecule.
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