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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:#000000;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 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>
</font></div>
<br>
<br>
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<br>
<a
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Éric Germaneau</a><br>
<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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