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<p>Here is a link towards a paper which treats such systems using
GGA and HSE06. Indeed, with GGA the band gap is nearly zero for
some of them. <br>
</p>
<p><a class="moz-txt-link-freetext" href="https://arxiv.org/pdf/1706.08674.pdf">https://arxiv.org/pdf/1706.08674.pdf</a></p>
<p>The point is that it seems to me that it will not be too much
problematic although your band gap is zero if the bands are simply
touching at the Fermi level. <br>
</p>
<p>You must check your band structure first. <br>
</p>
<p>In the paper they show HSEO6 band structures which clearly
confirm that GGA+U or onsite-hybrid will not be useful. <br>
</p>
<p>One option for you could be to do band structure using GGA and
Full-hybrid (HSE06 or PBE0) to compare and discuss the
differences. Then if the difference is only band gap reduction,
you could use GGA for the phonons calculations. In addition, it
will allow you to estimate the residual forces using both GGA and
hybrid, which will be one more confirmation (or not) that GGA is
sufficient for your system. <br>
</p>
<p>Cheers</p>
<p>Xavier<br>
</p>
<br>
<div class="moz-cite-prefix">Le 27/01/2018 à 16:01, chin Sabsu a
écrit :<br>
</div>
<blockquote
cite="mid:1717119010.1654471.1517065265086@mail.yahoo.com"
type="cite">Sir, it is A2BX6, x is halogen.<br>
<br>
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<div>On Sat, Jan 27, 2018 at 7:23 PM, Xavier Rocquefelte</div>
<div><a class="moz-txt-link-rfc2396E" href="mailto:xavier.rocquefelte@univ-rennes1.fr"><xavier.rocquefelte@univ-rennes1.fr></a> wrote:</div>
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