<div>Dear all,</div> <div> Thank you for all your help. I once considered the GW approximation.But what I calculate is heavy atoms,I can't deal with heavy atoms with the GW method.Maybe in the pseudo potential,can add the spin-orbit coupling.Is there indeed no other ways to calculate the small gap in wien2k?</div> <div> Thank you for your attention.</div> <div> Mary <BR><BR><B><I>Hong Jiang <jiang@fhi-berlin.mpg.de></I></B> wrote:</div> <BLOCKQUOTE class=replbq style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px
solid">Hi,<BR>Your problem is actually not related to which structure you use. <BR>Usually the band gap calculated from LDA or GGA can not be taken too <BR>seriously because of the well-known band gap problem of LDA/GGA, i.e. <BR>the LDA/GGA band gaps are always under-estimated, and therefore for <BR>small gap systems, LDA/GGA usually predict metallic behavior; for <BR>transition metal oxides like MnO, FeO, CoO and NiO, even experimental <BR>gaps are not small (~4 eV), LDA/GGA can also give zero or very small <BR>gaps. In terms of the band gap, EV93 is known to give larger gaps, but <BR>usually not large enough compared to experiment. For accurate and <BR>reliable calculations of band gaps, one has to use the GW <BR>approximation, which is unfortunately very expensive and only feasible <BR>for small systems. See the review paper in Rep. Prog. Phys. 61, 237 <BR>(1998) for more details.<BR>Hope this is helpful.<BR><BR>Hong<BR><BR><BR><BR><BR><BR>Mary White wrote:<BR>>
Dear R.Ahmed and all users,<BR>> Thank you for your attention. After I first optimized the <BR>> experimental structure (both lattice parameters and atom position), I <BR>> calculated the band structure and got the gap of 0.2ev with EV-GGA <BR>> (0.1ev with the experimental structure EV-GGA). The optimized lattice <BR>> parameters is a little overestimated compared the experimental lattice <BR>> parameters. But I found the force on the two atoms is a little large. <BR>> So I optimized the atom position in min_lapw (not for lattice <BR>> parameters).The force is small. Then I calculate the band gap with the <BR>> new optimized structure, but very strange,the gap is disappear. That <BR>> appears metallic. I don't know what I can do. Only optimized the atom <BR>> position,the result is very different (compared with the first <BR>> optimized structure 0.2ev).I think the optimized structure is ok.<BR>> There is no experimental band
structure. I don't know how to <BR>> deal with it. I tried to adjust the parameters,but that is no effect.<BR>> Any comments do help me.Thanks in advance.<BR>><BR>> */Rashid Ahmed <RASOFI@HOTMAIL.COM>/* wrote:<BR>><BR>> Dear Mary<BR>> hi<BR>> This is known fact that GGA usually overestimates lattice<BR>> parameters which susequently decrease band gap value. As your<BR>> result with experimental lattice parameters is already very small.<BR>> so with further decrease in band gap, it shows wrong mettalic band<BR>> structure.<BR>> bye<BR>> R. Ahmed<BR>><BR>> ------------------------------------------------------------------------<BR>> From: /Mary White <US.WHITE@YAHOO.COM>/<BR>> Reply-To: /A Mailing list for WIEN2k users<BR>> <WIEN@ZEUS.THEOCHEM.TUWIEN.AC.AT>/<BR>> To: /wien2k <WIEN@ZEUS.THEOCHEM.TUWIEN.AC.AT>/<BR>> Subject: /[Wien] band gap with EV-GGA and PBE-GGA/<BR>> Date: /Tue, 22 May 2007 05:10:18 -0700
(PDT)/<BR>><BR>> Dear all users,<BR>> <BR>> I calculated the energy band with EV-GGA and PBE-GGA.<BR>> The system is semiconductor.And the gap I calculated with the<BR>> experimental structure is about 0.1ev with EV-GGA.But when I<BR>> calculate with the optimized structure,the gap is<BR>> disappear.From the band structure,it appears<BR>> metallic.The band from EV-GGA and PBE-GGA seems very<BR>> similar.The parameters in the calculation with experimental<BR>> structure is the same as that with optimized structure.I think<BR>> the optimized structure is OK. I am puzzled with that.<BR>> <BR>> Does anyone know it? Look forward to your advice. Any<BR>> help will be appreciated.Thank you in advance.<BR>> <BR>> <BR>> <BR>> Mary White<BR>> <BR>> ------------------------------------------------------------------------<BR>> Moody friends. Drama queens. Your life? Nope! - their life,<BR>> your story.<BR>> Play
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