<div dir="ltr">The reason of negative frequencies are :<div>1. Look you pseudopotential perhaps use another and see.</div><div>2. Your structure is not well optimized.</div><div><br></div><div>and many...</div><div><br></div><div>wishes</div><div>sanjeev</div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Sun, Dec 13, 2015 at 8:02 PM, Barnali Bhattacharya <span dir="ltr"><<a href="mailto:barnalidgbhatt@gmail.com" target="_blank">barnalidgbhatt@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">
<p class="MsoNormal">Dear Sir/Madam</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal">I am a quantum espresso
user and interested in phonon calculation of BN doped two dimensional system.
For these first of all I have checked the <span style="font-family:"Courier New"">ecut
off convergence and taking this ecutwfc value checked the K-point convergence. After
taking this ecutwfc and k-point value, I have optimized the structure with ‘VC-relax
option’ with convergence criteria </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">“etot_conv_thr = 1.0D-16,</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""><span> </span>forc_conv_thr = 1.0D-16,”.</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Taking the relaxed position and cell parameter I have checked
the suitable lattice constant for which energy is minimum by using the script…</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">……………………………………………</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">#!/bin/sh</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………….</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">for a in `seq -w 0.995 0.001 1.005 `<span> </span></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">do</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………………………..</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">……………….</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">cat > $MOL.$<a href="http://a.in" target="_blank">a.in</a> << EOF</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">&control</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""><span> </span>calculation='relax',</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""><span>
</span>restart_mode='from_scratch',</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">………………….</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">……………….</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">……………..</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………………</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">/</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">CELL_PARAMETERS alat</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">`echo $a*12.901074275 | bc -l` 0.000000000<span> </span>0.000000000</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">`echo $a*6.45053713| bc -l`<span>
</span>`echo $a*11.1726581| bc -l` 0.00000</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""><span> </span>0.000000000<span> </span>0.000000000<span>
</span>29.000000000</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">ATOMIC_SPECIES</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">C 12.0107 C.pz-n-rrkjus_psl.0.1.UPF</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">ATOMIC_POSITIONS (crystal)</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">.............................</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">……………………………………..</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………………………………..</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">………………………………</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………………………………</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">…………………………………..</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">……………………………………………………………………………………………………………………………………………………………………</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Then taking the lattice constant for which energy is minimum and
the atomic position we have calculated phonon dispersion curve, but found negative
frequency.</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">I am confused is there any mistake in my procedure.<span> </span></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Could anyone please guide me how this negative frequency will be
disappear? Any help will be thankfully acknowledged.</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Thanking you.</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Sincerely</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Barnali Bhattacharya</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Ph.D student</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Department of physics</span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New"">Assam University </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;line-height:normal"><span style="font-family:"Courier New""> </span></p>
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