From owner-chemistry@ccl.net Wed Jun 13 13:44:01 2007 From: "Pablo Denis pablod(_)fq.edu.uy" To: CCL Subject: CCL:G: Limitations of DFT Message-Id: <-34476-070613133336-2616-UZ1TQc86t5Y7AiMc9n8K8w===server.ccl.net> X-Original-From: "Pablo Denis" Content-Transfer-Encoding: 8bit Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original Date: Wed, 13 Jun 2007 13:45:52 -0300 MIME-Version: 1.0 Sent to CCL by: "Pablo Denis" [pablod .. fq.edu.uy] Maybe the following references may help you JACS118-6036 concerted and stpewise mechanisms for DA reaction fo butadiene and ethylene and more or less the same for the stones wales transformation in C60 JACS125-5572 DFT has been used in both cases and for diradicals TS (JACS articles!!!)... Good luck, Pablo ----- Original Message ----- > From: "Shobe, David David.Shobe.#.sud-chemie.com" To: "Denis, Pablo Andres " Sent: Wednesday, June 13, 2007 11:48 AM Subject: CCL:G: Limitations of DFT > > 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. > If you are not an addressee or otherwise authorized to receive this > message, you should not use, copy, disclose or take any action based on > this e-mail or any information contained in the message. If you have > received this material in error, please advise the sender immediately by > reply e-mail and delete this message. > Thank you.> > > > > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.5.472 / Virus Database: 269.8.15/847 - Release Date: 12/06/2007 > 09:42 p.m. >