CCL:G: Orbital energies for neutral and cation species in vacuum and solution

 Sent to CCL by: "Johannes Hachmann" [jh^-^chemistry.harvard.edu]
 Hi Scott,
 The decision between U and RO is tricky and in my opinion there is no golden
 bullet - both approaches have their problems :). The U results may not give
 you a good quantum state, in particular if you have excessive spin
 contamination, while RO is not very flexible and does not allow for DODS,
 i.e., the R part cannot adequately account for the interactions with the
 unpaired electron. In my experience, U results tend to be better, but one
 has to be more careful how to interpret them. If you have very large spin
 polarization this indicates that your electronic structure is too
 complicated for a single-reference approach and you may want to try
 something more sophisticated than DFT. All bets are off then for the DFT
 results: you know that RO will be bad, while U results *may* still be ok
 (but somewhat artificially so).
 If you use U for the cation, you should try using U for the neutral molecule
 as well (don't forget to break spin symmetry in your guess!). If the
 molecule is complicated and the neutral one already shows spin polarization,
 you *may* get some error cancellation, since both species are treated on the
 same footing. I would then also use U for the respective geometry
 optimizations.
 Hope this helps
 Best
 Johannes
 -----------------------------------------------
 Dr. Johannes Hachmann
 Research Associate
 Harvard University
 Department of Chemistry and Chemical Biology
 12 Oxford St, Rm M104A
 Cambridge, MA 02138
 -----------------------------------------------
 > -----Original Message-----
 > From: owner-chemistry+jh==chemistry.harvard.edu++ccl.net [mailto:owner-
 > chemistry+jh==chemistry.harvard.edu++ccl.net] On Behalf Of Scott McKechnie
 > jsm78]![cam.ac.uk
 > Sent: Thursday, 16 May, 2013 07:28
 > To: Hachmann, Johannes
 > Subject: CCL:G: Orbital energies for neutral and cation species in vacuum
 and
 > solution
 >
 > Dear Dr. Hachmann and Dr. Kumar,
 >
 >
 > Many thanks for your very helpful replies and for the excellent
 references.
 >
 > >  Did you set up your delta-SCF as restricted open shell or
 unrestricted?
 >
 > I used the default settings in Gaussian09 which is unrestricted for the
 cation,
 > but I would appreciate any advise here. I notice that the delta SCF values
 only
 > differ by 0.07 eV but the orbital energies are significantly different
 (cation gas
 > phase alpha HOMO and LUMO values of -6.48 eV and -6.36 eV as compared
 > with the unrestricted beta gas phase HOMO and LUMO values of -10.65 eV and
 > -8.76 eV).
 >
 >
 > I had two follow up questions here.
 >
 >
 > 1. Using ROB3LYP results in degenerate alpha and beta orbitals?
 >
 >
 > 2. Is it advised to do cation optimization with UB3LYP instead of B3LYP?
 >
 >
 > Thanks again and very best wishes,
 >
 > Scott
 >
 >
 >
 >
 > On 15 May 2013 04:30, Ramesh Kumar rameshchitumalla^gmail.com <owner-
 > chemistry,+,ccl.net> wrote:
 >
 >
 >
 > 	Sent to CCL by: Ramesh Kumar [rameshchitumalla(a)gmail.com]
 > 	Dear Scott,
 >
 > 	The reply from Dr. Johannes Hachmann is really very useful.
 > 	The orbital energy of a cation is much lower than that of its
 > 	corresponding neutral molecule and again it comes  very near to the
 > 	case similar to neutral molecule in the presence of a polar solvent.
 > 	For some numbers you can see CPL 396 (2004) 43-48.
 >
 >
 >
 > 	--
 > 	With Best Regards:
 >
 > 	CH. Ramesh Kumar
 > 	Senior Research Fellow,
 > 	Computational Chemistry Lab,
 > 	Indian Institute of Chemical Technology(IICT),
 > 	Tarnaka,
 > 	Hyderabad.>
 >
 >
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 Conferences:
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 >
 >
 >