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Laptop?! That's my me the point ...<br>
Also usually people use a supercell to compute phonon.<br>
<br>
On 10/10/2011 01:47 AM, GAO Zhe wrote:
<blockquote
cite="mid:4c56c9c3.10ca1.132ec5f562b.Coremail.flux_ray12@163.com"
type="cite">
<div
style="line-height:1.7;color:#000000;font-size:14px;font-family:arial">Dear
Eric,<br>
Thank you very much for your reply.<br>
I haven't calculated graphite, and when I tried to calculate it,
I was in dormitory, at where I cannot download any paper......<br>
After reading your suggestion, I searched a paper publised at
Solid State Communications 131, 141-152, it clearly showed that
the phonon dispersion of graphite is stable.<br>
I think I may use too small cut-off and k-points due to my poor
performance of laptop~ I will try it, again, for a better
relaxation and result.<br>
<br>
<div>--<br>
GAO Zhe<br>
CMC Lab, MSE, SNU, Seoul, S.Korea<br>
</div>
<br>
At 2011-10-11
01:05:44,"Éric Germaneau" <a class="moz-txt-link-rfc2396E" href="mailto:germaneau@gucas.ac.cn"><germaneau@gucas.ac.cn></a> wrote:<br>
<blockquote id="isReplyContent" style="padding-left: 1ex;
margin: 0px 0px 0px 0.8ex; border-left: 1px solid rgb(204,
204, 204);"> Hey Gao,<br>
<br>
Well, I have a naive question/reply.<br>
<br>
Why do you get imaginary frequencies?<br>
<br>
I'd be very surprised of graphite instability at 0K ...<br>
Is you structure relaxed enough?<br>
<br>
This structure has been so much studied, have you ever read
such graphite properties before?<br>
<br>
May experts can tell us more about it.<br>
<br>
Éric.<br>
<br>
On 10/10/2011 12:37 AM, GAO Zhe wrote:
<blockquote
cite="mid:40ba7cab.100b0.132ec1f2d69.Coremail.flux_ray12@163.com"
type="cite">
<div style="line-height: 1.7; color: rgb(0, 0, 0);
font-size: 14px; font-family: arial;">Thank you for your
kindly reply.<br>
Well, normally, if we found imaginary frequency in phonon
dispersion, it indicates that this structure is not stable
or, at least, unstable at 0K. Therefore, the vibrational
free energy becomes unmeaningful because of the imaginary
value.<br>
However, as we know, graphite is stable phase in a wild
temperature range, including 0K. And when we discuss about
formation free energy of carbide at specified temperature,
graphite is used as reference pure material instead of
diamond. This means that the free energy of graphite
really exists.<br>
Therefore, I wondered whether there are some methods, for
example, improving the graphite model, can deal with this
problem and calculate vibrational free energy of graphite.
Or, whether it is possible that we can <span
class="enfont">neglect the vibration including
z-direction and only consider vibration in xy-plane.</span><br>
<br>
<div>--<br>
GAO Zhe<br>
CMC Lab, MSE , SNU, Seoul, S.Korea<br>
</div>
<br>
At 2011-10-09 20:52:02,"bhabya sahoo" <a
moz-do-not-send="true" class="moz-txt-link-rfc2396E"
href="mailto:bdslipun@gmail.com"><bdslipun@gmail.com></a>
wrote:<br>
<blockquote id="isReplyContent" style="padding-left: 1ex;
margin: 0px 0px 0px 0.8ex; border-left: 1px solid
rgb(204, 204, 204);">no because it is not valid for
negative frequncy in phonon spectrum
<div><br>
<br>
<div class="gmail_quote">2011/10/9 GAO Zhe <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:flux_ray12@163.com">flux_ray12@163.com</a>></span><br>
<blockquote class="gmail_quote" style="margin: 0pt
0pt 0pt 0.8ex; border-left: 1px solid rgb(204,
204, 204); padding-left: 1ex;">
<div style="line-height: 1.7; color: rgb(0, 0, 0);
font-size: 14px; font-family: arial;">Dear QE
developer and users:<br>
I am using QE 4.3.2 to calculate phonon and
thermaldynamis properties of materials. These
days, I need to find the Gibbs (or Helmholtz)
Free energy of carbon. Thus, I compared diamond
and graphite phases at 0K. As a result, graphite
provides lower energy per carbon atom (about
0.3eV/atom lower than of diamond).<br>
However, the phonon dispersion result shows that
the imaginary frequency occurred at Gamma, H, A
and K, so it is impossible by using QHA to find
the vibrational free energy at specified
temperature.<br>
The graphite model I used is:<br>
<i><span style="color: rgb(0, 0, 255);"><span
style="color: rgb(0, 0, 255);">ibrav = 0 ,</span><br>
<span style="color: rgb(0, 0, 255);">celldm(1)
= 1.889725989 ,</span><br>
<span style="color: rgb(0, 0, 255);">CELL_PARAMETERS</span><br>
<span style="color: rgb(0, 0, 255);">
2.130421558318710
-1.229999460182690 0.000000000000000</span><br>
<span style="color: rgb(0, 0, 255);">
0.000000000000000
2.459998920365380 0.000000000000000</span><br>
<span style="color: rgb(0, 0, 255);">
0.000000000000000
0.000000000000000 6.800000000000000</span><br>
<span style="color: rgb(0, 0, 255);">ATOMIC_POSITIONS
crystal</span><br>
<span style="color: rgb(0, 0, 255);"> C
0.0000000000000000 0.0000000000000000
0.2500000000000000</span><br>
<span style="color: rgb(0, 0, 255);"> C
0.0000000000000000 0.0000000000000000
0.7500000000000000</span><br>
<span style="color: rgb(0, 0, 255);"> C
0.3333333333333330 0.6666666666666669
0.2500000000000000</span><br>
<span style="color: rgb(0, 0, 255);"> C
-0.3333333333333330 -0.6666666666666669
0.7500000000000000</span></span></i><br>
Is there any other methods or models can
calculate the free energy of graphite at
specified temperature?<br>
Any suggestion will be welcome.<br>
Thanks.<br>
<font color="#888888"><br>
<div>--<br>
GAO Zhe<br>
CMC Lab, MSE, SNU, Seoul, S.Korea<br>
</div>
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<a
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<br>
Graduate University of Chinese Academy of Sciences<br>
College of Physical Sciences<br>
Yuquan Road 19A<br>
Beijing 100049<br>
China<br>
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