*From*: "Shobe, David" <David.Shobe]-[sud-chemie.com>*Subject*: CCL:G: Limitations of DFT*Date*: Wed, 13 Jun 2007 16:48:43 +0200

Sent to CCL by: "Shobe, David" [David.Shobe+/-sud-chemie.com] The real problem with DFT is as follows. The spin-restricted DFT (RDFT, as in Wai-To's response) FORCES pairing of every electron! If you have a diradical, RDFT will force the two unpaired electrons into the same orbital, which may or may not resemble either of the singly occupied orbitals in the diradical. And you may be using RDFT without being aware of it, because quantum-chemical programs in general use RDFT for any singlet species, unless you specify that UDFT (spin-unrestricted DFT) is to be used. Several months ago on CCL, there was a discussion on DFT (B3LYP?) calculations on the twisting of ethylene, and it may be helpful to review that discussion. The 90° twisted ethylene is very poorly described by DFT, because the two electrons are forced into one of the orthogonal p orbitals. Explicitly correlated methods such as CCSD or MP2 may be less vulnerable to this effect, because even though they rely on a single-determinant reference, they at least add in other determinants. Ideally one would use a multi-determinant reference method for problems like this, but they are not easy to use! --David Shobe -----Original Message----- > From: owner-chemistry+/-ccl.net [mailto:owner-chemistry+/-ccl.net] Sent: Wednesday, June 13, 2007 7:45 AM To: Shobe, David Subject: CCL:G: Limitations of DFT Sent to CCL by: Wai-To Chan [chan++curl.gkcl.yorku.ca] <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< However, one of the reviewers on *this* manuscript objected to the use of the DFT calculations with the following: "With regards to using DFT calculations to distinguish between mechanisms B and C, I would put no confidence in such a result. It is well known that DFT calculations greatly overemphasize the stabilization provided by delocalization, so they have a built-in bias towards concerted reactions, which is useful if the reaction actually is concerted, but a disaster if it is not." Is this a true statement? And, if so, where can I find more about this limitation of DFT theory? I have searched but come up empty so far. Plus, I'm not sure exactly what the reviewer means, since the diradical intermediate of mechanism C would also be subject to delocalization effects...possibly even greater than those expected by a "concerted" reaction! >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> I assume reaction pathway (B) proceeds via a TS characterized by spin-restricted (R)DFT, (C) a open-shell TS characterized by unrestricted DFT and the reactant is closed-shell and treated by RDFT. Did you check the wavefunction stability of the TS for (B)? Instability of RHF and/or RDFT is a telling sign of discontinuity within your PES. The theoretical foundation underlying the characterization of the kind of PES with stationary points of varying degree of diradical character using single-reference type of methods is shaky. I however don't see any relevance of your reviewer's remarks to the limiation of DFT. DFT indeed does suffer from wavefunction stability and its reliability for the prediction of concerted and stepwise mechanism involving diradicals is seldom addressed. But I never heard of any studies which demonstrate that DFT is biased towards the concerted pathway. I seem to remember that DFT has been applied to characterization of the aromaticity of transition state structures in concerted reaction mechanism through NICS calculations. I am not familiar with the literature in this area but I doubt very much one can draw any conclusion from such studies alone on DFT's favouring of the concerted mehanism in pericyclic reactions. If I were you I would demand the referee to provide a reference to back up his/her comments further. Chance is that the referee's comments are merely speculations rather than established facts. It is not just DFT. The entire specturm of commonly employed single-reference type of methods (HF, MPn, CCSD(T), DFT) is inherently problematic for a consistently accurate characterization of a diradical PES. CASSCF in principle could solve the problem arising > from PES discontinuity. In practice CASSCF won't necessarily give more accurate structural and energetic predictions than DFT. DFT has been applied to the study of concerted and stepwise mechansims in cycloaddition reaction for years. Occassionaly serious discrepancies between DFT and CASSCF were noted. But no one to my knowledge has suggested DFT should be avoided in favour of other methods in general. There are other promising alternatives to DFT for diradical PES outside GAUSSIAN. Unless these methods have already gained general acceptance and are applicable to the system you are studying you don't have the obligation to 'upgrade' your DFT calculations in my view. My advice for you is to come up with a cogent response to the questions of the referee. You should in your discussion of your results highlight those limitations of the theories employed and convince the readers that the computations were properly executed within the limitations. As long as you lay out the facts clearly to allow the readers to assess your results you should stand a reasonable chance of getting your work accepted. Wai-To Chanhttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txtThis e-mail message may contain confidential and / or privileged information. 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