Dear Prof. Kohlmeyer: Thank you very much for your kind attention! And I appreciate your patient instruction very much! Best Wishes! Yours Sincerely L. F. Huang
> From: Axel Kohlmeyer > Subject: Re: [Pw_forum] about the quantum tunneling of diffusing atoms > To: PWSCF Forum > Message-ID: > Content-Type: text/plain; charset="UTF-8" > > On Tue, 2009-03-31 at 08:07 +0200, Stefano Baroni wrote: > > Dear LF Huang, > > > > > > no code will ever be a substitute of common sense. What you need is > > simply the potential energy (i.e. "total energy" in the usual DFT > > parlance) of a system, as a function of the coordinates of the > > diffusing atom. As simple (or as complicated) as that! > > please let me add my 2 cents to this. > > you can go back to a quantum mechanics text book and look up > for example the discussions of quantum particle vs. wall cases. > the potential doesn't change whether the particle is quantum > or classical! > > what you seem to be looking for is some kind of > "barrier crossing probability". now, wrt to that i'd have several > concerns: > > - how accurate is your "classical" barrier potential to begin with? > you are doing graphite and hydrogen and use plain DFT. the > interaction between a benzene molecule and a hydrogen molecule is > a frequently used test case for methods that add dispersion > interactions corrections to DFT. hmmm... > > - is tunneling relevant at all? at T > 0K the carbon atoms move and > your barrier will fluctuate, that will affect the crossing > probability. similarly, if your hydrogen has enough kinetic energy, > tunneling is irrelevant. > > - what is the correlation length of your system? only if it is long > quantum effects of the atom core matter. since you seem to be > doing a solid state vacuum system, you should be good on that. > > after you've made sure that all of the above is not rendering any > further studies of the quantum effects pointless, _then_ i would look > into path-integral methods (e.g. the works of mark tuckerman and dominik > marx) that allow studying probability distributions at finite > temperature, albeit mostly in imaginary time. mind you, those > calculations are hugely expensive and you may be best off to first > make some tests with a classical potential. in fact, i would not > be surprised if a suitably chosen classical potential would give > you a better representation of the potential barrier than DFT. > > cheers, > axel. ------ ====================================================================== L.F.Huang(???) lfhuang at theory.issp.ac.cn ====================================================================== Add: Research Laboratory for Computational Materials Sciences, Instutue of Solid State Physics,the Chinese Academy of Sciences, P.O.Box 1129, Hefei 230031, P.R.China Tel: 86-551-5591464-328(office) Fax: 86-551-5591434 Web: http://theory.issp.ac.cn (website of our theory group) http://www.issp.ac.cn (website of our institute) ====================================================================== -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20090401/85a2c7d2/attachment.htm
