<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 >diamond.scf.in << 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
< diamond.scf.in > diamond.scf.out<BR>$ECHO " done"</div> <div># phonon calculation at G<BR>cat >diamond.phG.in << EOF<BR>phonons of C at Gamma<BR> &inputph<BR> tr2_ph=1.0d-14,<BR> epsil=.true.,<BR> prefix='diamond',<BR> fildyn='diamond_g.dyn',<BR> amass(1)=12.0107,<BR> outdir='$TMP_DIR/'<BR> /<BR>0.0 0.0 0.0<BR>EOF<BR>$ECHO " running the phonon calculation at Gamma...\c"<BR>$PH_COMMAND < diamond.phG.in > diamond.phG.out<BR>$ECHO " done"</div> <div>$ECHO<BR>$ECHO "$EXAMPLE_DIR: done"<BR>================================================</div><p>
<hr size=1><a href="http://cn.mail.yahoo.com" target=blank>ÇÀ×¢ÑÅ»¢Ãâ·ÑÓÊÏä-3.5GÈÝÁ¿£¬20M¸½¼þ£¡</a>