<div>Dear Stefaan,</div> <div>Dear Laurence,</div> <div> </div> <div>Thank you very much for your useful and extended replies. I was just repeating my pseudo-potential calculations by this code by taking the number of k-points as minimum as possible. Now, by your comments I learn that these two minimization methods would agree (within at most 0.1 mRy/au) only for sufficiently large number of k points.</div> <div>Thank you again for clarifying this point.</div> <div> </div> <div> Best Regards.<BR> Nasser Zare Dehnavi<BR><BR><B><I>Stefaan Cottenier <Stefaan.Cottenier@fys.kuleuven.be></I></B> wrote:</div> <BLOCKQUOTE class=replbq style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px solid"><BR>> 3)I use RK=7 and mesh 18X18X18 for bulk and 10X10X1 for <BR>> asupercell with 38 atoms and a surface cell(2X2) .I study about <BR>> work function and I use from this parameters because :a)give me good <BR>>
value for workfunction and b)for avoid of computational cost <BR>> of large parameters.<BR><BR>Don't take this personally, but these are very bad and even dangerous <BR>arguments. As computational physicists, we MUST NOT take the <BR>parameters that give us good agreement with experiment. We must take <BR>the parameters that give us numerically accurate results within the <BR>framework we work in (e.g. DFT-LDA). If we don't do that, we're just <BR>doing meaningless fitting, not physics. If it turns out you find the <BR>computational cost is too much for the quantity you want to study, <BR>then so be it. Put the problem aside and do something else instead of <BR>doing something meaningless. In your case, this means you must verify <BR>whether going to a larger RKmax or fine k-mesh does not change you <BR>work function to an amount that would be large enough to alter your <BR>physical conclusions. As long as you are not sure about this, there is <BR>no point in proceeding
with the rest of your work.<BR><BR>There is a good reason why I spent some time writing this down: a bad <BR>approach to DFT studies affects the entire community. Imagine you come <BR>to some conclusion based on an unjustified small basis set. Perhaps <BR>you'll even make a prediction based on your results. Then an <BR>experimentalist tries to verify your prediction, and find it to be <BR>wrong. Obviously, the conclusion of his or her paper will be: DFT <BR>gives bad predictions. If this happens too often, then DFT gets an <BR>unjustified bad image, and few people will believe DFT predictions in <BR>the future, even if they are made with good basis sets. Our entire <BR>community has a responsibility in keeping up the credibility of DFT: <BR>there are enough places where it doesn't agree with experiment, we <BR>should not create additional ones by being too tolerant on numerical <BR>issues as basis set and k-mesh.<BR><BR>I hope you seriously think about this before
continuing!<BR><BR>Now back to your problem:<BR><BR>As you apparently did not test the accuracy of your forces with <BR>respect to k-mesh and basis set, it might be that you are working with <BR>too much numerical noise on the forces, leading mini to an incorrect <BR>result. So test the accuracy of your forces.<BR><BR>This is an interesting issue:<BR><BR>> with attention to this point that min-energy is obtained in <BR>> smaller time by plot total energy versus distance of atom from <BR>> surafce with respect to use of mini..<BR><BR>As the force is the gradient of the total energy surface, this <BR>statement cannot be true. Even if you don't use mini and displace the <BR>atoms yourself, it is worthwhile to calculate the force in the last <BR>iteration and plot not :ENE but the force as a function of <BR>displacement. It will show you much better in which direction and by <BR>which amount you have to displace, requiring less steps to find the <BR>equilibrium than
if you use only the energy information. As a check, <BR>of course, you can look at the energies afterwards. If your numerical <BR>precision was OK, the zero of the force and the energy minimum must <BR>agree.<BR><BR>Stefaan<BR><BR><BR>Quoting naser zare <NASER_ZD2000@YAHOO.COM>:<BR><BR>> Dear Stefaan,<BR>> Thank you very much for your reply.<BR>> I should mention that:<BR>> 1)the discrepancy in position is 0.122A(1.544A for min-energy and <BR>> 1.666 A for zero-force)<BR>> 2)convergence criteria for energy is 0.0001ry and for charge is <BR>> 0.002 and when I use from<BR>> mini for force is 4 mry/au<BR>> 3)I use RK=7 and mesh 18X18X18 for bulk and 10X10X1 for <BR>> asupercell with 38 atoms and a surface cell(2X2) .I study about <BR>> work function and I use from this parameters because :a)give me good <BR>> value for workfunction and b)for avoid of computational cost <BR>> of large parameters.<BR>> 4)when I used mini , I move only
one atom ( like when I obtain <BR>> min of energy ) and all atoms of slab are fixed.<BR>><BR>> I have a question,can I fell sure that point obtained from plot <BR>> of total energy versus distance of atom from surafce is true <BR>> point and my problem is calculation of force?with attention to this <BR>> point that min-energy is obtained in smaller time by plot total <BR>> energy versus distance of atom from surafce with respect to use <BR>> of mini..<BR>><BR>> thank you very much for your attention.<BR>> Best Regards.<BR>> Nasser Zare Dehnavi<BR>><BR>><BR>> Stefaan Cottenier <STEFAAN.COTTENIER@FYS.KULEUVEN.BE>wrote:<BR>><BR>> You need to provide more information, for instance:<BR>><BR>> * How big is the discrepancy in position?<BR>> * How well are your calculations converged and what are your<BR>> convergence criteria?<BR>> * Did you verify your basis set and k-mesh are well enough to provide<BR>> good
forces?<BR>> * When you used mini, did you move only one atom, or all of them?<BR>> *...<BR>><BR>> Stefaan<BR>><BR>><BR>><BR>> Quoting naser zare :<BR>><BR>>> Hello<BR>>> Dear all:<BR>>> I use from Wien2k 6.4. I want to obtain position of adsorption of<BR>>> atom on a surface.I do it in two way :1)I move atom towards<BR>>> surface step by step and I plot total energy<BR>>> versus distance from surface and I obtain point of min energy and<BR>>> 2)I relax position of atom by MINI, but these two point are not<BR>>> the same.on the other hand ,point that obtained by Mini has not<BR>>> lowest energy. I want to know reason of this problem .thank you<BR>>> very much for your attention.<BR>>> Best Regards.<BR>>> Nasser Zare Dehnavi<BR>>><BR>>><BR>>><BR>>><BR>>> ---------------------------------<BR>>> Sucker-punch spam with award-winning
protection.<BR>>> Try the free Yahoo! Mail Beta.<BR>><BR>><BR>><BR>> --<BR>> Stefaan Cottenier<BR>> Instituut voor Kern- en Stralingsfysica<BR>> K.U.Leuven<BR>> Celestijnenlaan 200 D<BR>> B-3001 Leuven (Belgium)<BR>><BR>> tel: + 32 16 32 71 45<BR>> fax: + 32 16 32 79 85<BR>> e-mail: stefaan.cottenier@fys.kuleuven.be<BR>><BR>><BR>> Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm<BR>><BR>> _______________________________________________<BR>> Wien mailing list<BR>> Wien@zeus.theochem.tuwien.ac.at<BR>> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien<BR>><BR>><BR>><BR>> ---------------------------------<BR>> It's here! Your new message!<BR>> Get new email alerts with the free Yahoo! Toolbar.<BR><BR><BR><BR>-- <BR>Stefaan Cottenier<BR>Instituut voor Kern- en Stralingsfysica<BR>K.U.Leuven<BR>Celestijnenlaan 200 D<BR>B-3001 Leuven (Belgium)<BR><BR>tel: + 32 16 32 71
45<BR>fax: + 32 16 32 79 85<BR>e-mail: stefaan.cottenier@fys.kuleuven.be<BR><BR><BR>Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm<BR><BR>_______________________________________________<BR>Wien mailing list<BR>Wien@zeus.theochem.tuwien.ac.at<BR>http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien<BR></BLOCKQUOTE><BR><p> 
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