I have done such a study of biaxial strain on anatase titania to find the derivative of the band gap using both DFT and GW approximation. For small strain, the error between the two methods was ~5% and grows rapidly to ~15% as the strain increases. Maybe this information is useful.<br>
<br>
Luke Thulin<br>
Nanoptek Corp.<br>
Maynard, MA<br>
<br>
<br>
-----Original Message-----<br>
From: Nicola Marzari <marzari@MIT.EDU><br>
To: PWSCF Forum <pw_forum@pwscf.org><br>
Sent: Thu, 27 Sep 2007 5:57 pm<br>
Subject: Re: [Pw_forum] Is Electronic-Enthalpy Functional for Finite Systems Under Pressure implemented in espresso-3.2?<br>
<br>
<div id=AOLMsgPart_1_e45fb8ff-ab18-4532-b0dd-383f804c04ea style="FONT-SIZE: 12px; MARGIN: 0px; COLOR: #000; FONT-FAMILY: Tahoma, Verdana, Arial, Sans-Serif; BACKGROUND-COLOR: #fff">Hongjun Xiang wrote: <br>
> Dear Dr. Cococcioni and others, <br>
> I tried some test calculations and realized that the pressure changes > the electronic structure and thus <br>
> decreases the volume of the system. If I am interested in the band gap > of a finite system under pressure, which method of the followings is > correct? <br>
> (1) Relax the structure under the pressure, and calculate the gap using > the relaxed structure under the pressure. <br>
> (2) Relax the structure under the pressure, and calculate the gap using > the relaxed structure without the pressure. <br>
> > It turns out the results from the above procedures are different. <br>
> > Thank you very much. <br>
> > Best regards, <br>
> Hongjun Xiang <br>
<br>
Dear Xiang, <br>
<br>
1) is the correct procedure - but be aware that the band gap is <br>
not a property that you can address in DFT - not in principle, and <br>
largely not in practice. At best, you could look at DFT predictions <br>
of, say, the derivative of the band gap in bulk semiconductors <br>
with respect to biaxial strain, and see how well you can do to predict <br>
the slope. I think the error would be in the 20-30% range, a bit better <br>
than the 100% error on the band gap. <br>
<br>
nicola <br>
<br>
--------------------------------------------------------------------- <br>
Prof Nicola Marzari Department of Materials Science and Engineering <br>
13-5066 MIT 77 Massachusetts Avenue Cambridge MA 02139-4307 USA <br>
tel 617.4522758 fax 2586534 <A href="mailto:marzari@mit.edu">marzari@mit.edu</A> <A href="http://quasiamore.mit.edu/" target=_blank>http://quasiamore.mit.edu</A> <br>
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