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
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