<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><div id="divNeteaseMailCard"></div><br>At 2011-10-09 20:52:02,"bhabya sahoo" <bdslipun@gmail.com> 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 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>
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