<P>Dear all pw users:</P>
<P>I'm using QE-5.0.1 to calculate electron-phonon interaction for a ternary system (ibrav=4) with Li, B, and C.<BR>But I get contradictory results for different k-grids. </P>
<P>Calculation details:<BR>I choose ecutwfc =40.0, ecutrho=480.0, combined with smearing = 'gaussian', degauss = 0.02 Ry.<BR>The lattice parameters have been relaxed for different k-grid and pseudo potentials to get <BR>the corresponding equilibrium point. For example, the lattice parameters for pbe US potentials <BR>are about a=2.84 angstrom and c=7.12 angstrom. I set conv_thr = 1.0d-14 and tr2_ph=1.0d-18 <BR>in the scf and phonon calculation, respectively.</P>
<P>I use three different groups of k-grids to test the calculated results.<BR>the first group: <BR>densh k-grid: 24x24x12<BR>coarse k-grid: 12x12x6<BR> q-grid: 8x8x4<BR>the second group:<BR>densh k-grid: 32x32x16<BR>coarse k-grid: 16x16x8<BR> q-grid: 8x8x4<BR>the third group:<BR>densh k-grid: 36x36x24<BR>coarse k-grid: 18x18x12<BR> q-grid: 6x6x4</P>
<P><BR>Different pseudo potentials are also tested.</P>
<P>PBE:<BR>Li 6.941 Li.pbe-s-van_ak.UPF<BR>C 12.011 C.pbe-van_ak.UPF<BR>B 10.81 B.pbe-n-van_ak.UPF</P>
<P>PW91:<BR>Li 6.941 Li.pw91-s-van_ak.UPF<BR>C 12.011 C.pw91-van_ak.UPF<BR>B 10.81 B.pw91-n-van.UPF</P>
<P>PZ (combined NCPP and USPP):<BR>Li 6.941 Li.pz-s-van_ak.UPF<BR>C 12.011 C.pz-van_ak.UPF<BR>B 10.81 B.pz-bhs.UPF</P>
<P>PZ-vbc:<BR>Li 6.941 Li.pz-n-vbc.UPF<BR>C 12.011 C.pz-vbc.UPF<BR>B 10.81 B.pz-vbc.UPF</P>
<P>Input file for electron-phonon interaction:</P>
<P> &inputph<BR> tr2_ph=1.0d-18,<BR> prefix='LiBC',<BR> fildvscf='LiBCdv',<BR> amass(1)=6.941,<BR> amass(2)=12.011,<BR> amass(3)=10.81<BR> outdir='/home/gm/work/tmp/',<BR> fildyn='LiBC.dyn',<BR> electron_phonon='interpolated'<BR> trans=.true.,<BR> ldisp=.true.<BR> nq1=6, nq2=6, nq3=4</P>
<P>My questions are as follows.</P>
<P>(1)The electron-phonon interaction parameter lambda is extremely large for the lowest phonon<BR>branch at some q points. For example for pbe-USPP, the largest lambda for the first group is 2.1725<BR>at q = ( 0.000000000 -0.577350269 0.099769209 ), for second group is 19.4839 at<BR>q = ( 0.125000000 0.360843918 0.000000000 ), and for third group is 3.2192 at<BR>q = ( 0.000000000 -0.577350269 0.099718425 ). And the similar behaviors for PZ-NCPP.<BR>Is the extremely large lambda for lowest phonon branch reasonable?<BR>(2)for the third group k-grid with PBE-USPP, when asr=simple, there is a small imaginary<BR>frequency about -8 cm^-1 around Gamma to A (0,0,pi/c). But when asr=crystal, there is no<BR>imaginary frequency along the high symmetry lines. While for PZ-PP, when asr=simple,<BR>there is no imaginary frequency along the high symmetry lines. So which result should I trust?<BR>(3) For the second group, after applid asr, there are lots of imaginary frequencies, with the<BR>maximum about -167 cm^-1 at q=(0.186667 0.323316 0.199453). The result contradict that of<BR>the third group. But the first group k-grid show similar behavior with the third group,<BR>why does the second group k-grid show different results?</P>
<P>Could anyone give me some clue? I am greatly indebted to you for your help. Thanks again!</P>
<P>Miao Gao<BR>------------------<BR>Ph.D. student of Department of Physics, <BR>Renmin University of China.<BR>------------------<BR><BR></P><SPAN></SPAN><br><br>