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:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div style="line-height:1.7;color:#000000;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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