Dear Niu,<br><br>Have you searched the mail postings for a similar topic?<br><br>Go to<br><a href="http://www.pwscf.org">www.pwscf.org</a><br><br>On the right hand panel, click on "Search Archive" under USERS FORUM. Search for

"pw_forum lattice constant" Do not know too much about phonon calculations, but in general they are much more delicate than a total energy calculations + structural minimization. Please note that the stress tensor converges much slower than than the total energy.

<br>For example, for a norm-conserving pseudopotential. If your energy is converged at 40 Ry,<br>the stress tensor may not converged (+/- 3 kbar) until you get to 80 Ry. <br><br><div><span class="gmail_quote">On 3/29/07, 

<b class="gmail_sendername">li niu</b> <<a href="mailto:niuli1978@yahoo.com.cn">niuli1978@yahoo.com.cn</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">

<div>Dear all,</div>  <div> I  have several questions on how to obtain a optimized lattice constant. I took the diamond as an  example, <br>  when: C.blyp-mt.UPF<br>           ecutrho = 4*ecutwfc<br>           a = 6.74 (experimental value)

<br>          K_POINTS AUTOMATIC  8 8 8 1 1 1<br>1.I did several scf and phonon calculations with different ecutwfc from 40 to 80 Ry and found a convergence of the calculated frequencies at 55 Ry (or 60).[Is it right?]</div>

  <div>====================================================<br>ecutwfc       total energy           P=              frequency( gamma

 point)<br>--------------------------------------------------------------------------------------------------<br>40 Ry           -22.54608438 Ry      -324.91          1364.516269 [cm-1]<br>---------------------------------------------------------------------------------------------------

<br>45             -22.58756098         P= -198.59      1356.978995 [cm-1]<br>---------------------------------------------------------------------------------------------<br>46             -22.59358560 Ry      P= -177.09

       1350.238444

 [cm-1]<br>-------------------------------------------------------------------------------------------------<br>48             -22.60333041         P= -139.82       1331.376654  (-11)<br>---------------------------------------------------------------------------------------------

<br>49             -22.60713824 Ry      P= -124.22       1320.862623    (-9)<br>-------------------------------------------------------------------------------------------------<br>50             -22.61073198            -

108.78       1311.304111    

 (9)<br>-------------------------------------------------------------------------------------------------<br>51             -22.61399076 Ry      P=  -94.25       1302.308894 [cm-1] (8)<br>-------------------------------------------------------------------------------------------------

<br>52             -22.61688998 Ry       P=  -80.83      1294.840278 [cm-1] (4)<br>-----------------------------------------------------------------------------------------------------<br>53             -22.61953601 Ry      P=  -

67.95       1290.058731 [cm-1] (5)

 <br>-----------------------------------------------------------------------------------------------------<br>54            -22.62200281          P=  -55.48       1285.954996 [cm-1]  (2)<br>------------------------------------------------------------------------------------------------------

<br>55            -22.62410759 Ry        P=  -44.35       1283.736877 [cm-1] (1)<br>-------------------------------------------------------------------------------------------------------<br>56            -22.62623466           P=  -

32.75       1282.451118 [cm-1]

 (0)<br>-------------------------------------------------------------------------<br>57            -22.62815534 Ry         P=  -21.82       1282.144868 [cm-1]  (0) <br>-------------------------------------------------------------------------

<br>58            -22.62986744 Ry         P=  -11.70       1282.755051 [cm-1]  (0)<br>-------------------------------------------------------------------------<br>59             -22.63154997 Ry        P=   -1.37        1284.145461

 [cm-1]

 (-2)<br>---------------------------------------------------------------------------<br>60          -22.63297919 Ry         P=    7.72       1285.831946 [cm-1]  (-1)<br>----------------------------------------------------------------------------------------------

<br>62            -22.63565052 Ry         P=   25.81       1289.073779 [cm-1]  (-4)<br>----------------------------------------------------------------------------------------------------<br>65         warning: symmetry operation #  5 not allowed.

<br>           -22.63882875 Ry          P=  

 49.83         1290.953300 [cm-1]<br>-----------------------------------------------------------------------------------------<br>70        warning: symmetry operation #  5 not allowed.  <br>          -22.64202864             P=   

78.96         1295.485613 [cm-1]<br>---------------------------------------------------------------------------------------------<br>80        warning: symmetry operation #  5 not allowed. <br>           -22.64415626 Ry         P=  

101.58         1293.336207 [cm-1]<br>======================================================</div>  <div>2.

 Then I did several calculations with different a (around experiment value)<br>when ecutwfc equal to 60 Ry. The results are follows.</div>  <div> </div>  <div>ecutwfc = 60<br>-------------------------------------------------------------------------------------------------------

<br>  a         total energy                            frequency(experimental:1332)<br>6.74        -22.63297919 Ry         P=    7.72       1285.831946 [cm-1] <br>6.75        -22.63324360 Ry         P=  -11.28       1279.754275

 [cm-1]<br>6.76       

 -22.63347229 Ry         P=  -29.78       1274.313572 [cm-1]<br>6.80        -22.63386503 Ry         P= -101.36       1252.534320 [cm-1]<br>6.85        -22.63347988 Ry         P= -183.28       1226.237110 [cm-1]<br>-----------------------------------------------------------------------------------------------------

</div>  <div>3. The potential energy surface is then fitted by the murnaghan equation (ev.x),<br>and its mininum provides the theoetical precictions for the ground state equilibrium<br>lattice constant.<br>--------------------------------------------------------------------------------------------------------

<br># equation of state: murnaghan.       

 chisq = 0.1134D-10<br># a0 =  6.80  k0 = 3746 kbar,  dk0 = 14.50  d2k0 =  0.000  emin =  -22.63387<br>--------------------------------------------------------------------------------------------------------</div>  <div>

4. In the end, I obtained optimized lattice constant is 6.8 when the energy cutoff is 60 Ry. In this case the phonon frequency of gamma piont equals to 1252 cm-1 but experimental is 1332 cm-1 and the stress-tensor is -101 kbar but not is 0.

<br>  <br>My questions are:<br>(1)Is it right to choose the energy cutoff like this? <br>(2) Why the stress-tensor is so large?<br>(3) How should I do to obtain the correct optimized parameters?</div>  <div> </div>  <div>

I'm puzzled by these questions all the times so I hope that you can help me.</div>  <div> </div>  <div>Thanks in advance!</div>  <div> </div>  <div>Bset!</div>  <div>Niu Liu<br><br>========================================================

<br>#

 self-consistent calculation<br>cat ><a href="http://diamond.scf.in" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">diamond.scf.in</a> << EOF<br> &control<br>    calculation='scf'

<br>    restart_mode='from_scratch',<br>    prefix='diamond',<br>    pseudo_dir = '$PSEUDO_DIR/',<br>    outdir='$TMP_DIR/'<br>    tstress = .true. ,<br>    tprnfor = .true. ,<br> /<br> &system    

<br>    ibrav=  2, celldm(1) =6.74, nat=  2, ntyp= 1,<br>    ecutwfc =60,    <br> /<br> &electrons<br>    conv_thr =  1.0d-9<br>    mixing_beta = 0.7<br> /<br>ATOMIC_SPECIES<br> C   12.0107  C.blyp-mt.UPF<br>ATOMIC_POSITIONS

<br> C  0.00 0.00 0.00 <br> C  0.25 0.25 0.25 <br>K_POINTS AUTOMATIC<br>8 8 8 1 1 1</div>  <div>EOF<br>$ECHO "  running the scf calculation...\c"<br>$PW_COMMAND

 < <a href="http://diamond.scf.in" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">diamond.scf.in</a> > diamond.scf.out<br>$ECHO " done"</div>  <div># phonon calculation at G<br>cat >

<a href="http://diamond.phG.in" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">diamond.phG.in</a> << EOF<br>phonons of C at Gamma<br> &inputph<br>  tr2_ph=1.0d-14,<br>  epsil=.true.,<br>  prefix='diamond',

<br>  fildyn='diamond_g.dyn',<br>  amass(1)=12.0107,<br>  outdir='$TMP_DIR/'<br> /<br>0.0 0.0 0.0<br>EOF<br>$ECHO "  running the phonon calculation at Gamma...\c"<br>$PH_COMMAND < <a href="http://diamond.phG.in" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">

diamond.phG.in</a> > diamond.phG.out<br>$ECHO " done"</div>  <div>$ECHO<br>$ECHO "$EXAMPLE_DIR: done"<br>================================================</div><p> 

                </p><hr size="1"><a href="http://cn.mail.yahoo.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">��ע�Ż��������-3.5G������20M������</a> <p></p></blockquote></div><br><br clear="all"><br>-- <br>Nichols A. Romero, 

Ph.D.<br>1613 Denise Dr. Apt. D<br>Forest Hill, MD 21050<br>443-567-8328 (C)<br>410-306-0709 (O)<br>