<div dir="ltr"><div><div><div><div>Hello all,<br><br></div>I am trying to calculate the phonon dispersion of a monolayer MXene structure, Ti3C2O2. My input files roughly follow the recipe laid out in the following publication: <b>Phys. Chem. Chem. Phys., 2015, 17, 9997--10003</b>, with the exception that I am trying to use Perturbation theory rather than a supercell approach.<b> </b>I have pasted my input files below for clarity. I am using QE-6.1. I start with a force relaxation calculation where the forces relax to<b>: 0.000039 Ry/bohr or ~0.001 eV/Ang</b>. After the structural relaxation, I perform a gamma-only phonon calculation to see if the gamma-point frequencies reproduce those shown the in publication above, and they more or less do. So I then move on to a full phonon dispersion calculation, where the dyn1 Gamma-point converges well (with conv_thr=1.0d-12) with reasonable frequencies (see below) but dyn2 does not converge unless I loosen the conv_thr to ~2.0d-8. In doing so, I get very erratic frequencies.<br></div><div><b><br></b></div><div><b>%% resulting forces and pressure</b><br></div><div> Forces acting on atoms (cartesian axes, Ry/au):<br><br> atom 1 type 1 force = 0.00000000 0.00000000 0.00002656<br> atom 2 type 1 force = 0.00000000 0.00000000 -0.00001506<br> atom 3 type 1 force = 0.00000000 0.00000000 -0.00001422<br> atom 4 type 2 force = 0.00000000 0.00000000 0.00001412<br> atom 5 type 2 force = 0.00000000 0.00000000 -0.00001214<br> atom 6 type 3 force = 0.00000000 0.00000000 -0.00000168<br> atom 7 type 3 force = 0.00000000 0.00000000 0.00000242<br><br> <b>Total force = 0.000039 </b> Total SCF correction = 0.000002<br><br><br> Computing stress (Cartesian axis) and pressure<br><br> total stress (Ry/bohr**3) (kbar) P= -3.79<br> -0.00000053 -0.00000000 0.00000000 -0.08 -0.00 0.00<br> -0.00000000 -0.00000053 0.00000000 -0.00 -0.08 0.00<br></div><div> 0.00000000 0.00000000 -0.00007620 0.00 0.00 <b> -11.21 !! is this alarmingly high? even though the forces look fine.<br></b></div><div><b><br></b></div><b>%% Relaxation input file that give the above forces<br></b></div><div>&control<br> calculation='vc-relax',<br> restart_mode='from_scratch',<br> outdir='./out_Ti3C2',<br> max_seconds=144000,<br> prefix='ti3c2o2_phonons',<br> pseudo_dir='/qe-6.1/pseudo',<br> forc_conv_thr=5.0D-5,<br> etot_conv_thr=5.0D-5,<br>/<br>&system<br> ibrav=0, celldm(1)=5.71020728999, !celldm(3)=6.120077,<br> nat=7, ntyp=3,<br> ecutwfc=40, ecutrho=320, occupations='smearing', smearing='methfessel-paxton',<br> degauss=0.007349862,<br>/<br>&electrons<br> electron_maxstep=250,<br> mixing_beta=0.7,<br> conv_thr=1.0d-12<br>/<br>&ions<br> ion_dynamics='bfgs',<br>/<br>&cell<br> cell_dynamics='bfgs',<br> cell_dofree='2Dxy',<br>/</div><div>ATOMIC_SPECIES</div><div> Ti 47.867 Ti.pbe-sp-van_ak.UPF</div><div> C 12.01 C.pbe-van_ak.UPF</div><div> O 15.999 O.pbe-van_ak.UPF</div><div>ATOMIC_POSITIONS {alat}<br>Ti 0.0000000000 0.0000000000 2.0371640580 1 1 1<br>Ti 0.0000000000 0.5773502692 2.8897518789 1 1 1<br>Ti 0.5000000000 0.288675135 1.1845717437 1 1 1<br>C 0.5000000000 0.288675135 2.4539683895 1 1 1<br>C 0.0000000000 0.5773502692 1.6203671083 1 1 1<br>O 0.0000000000 0.0000000000 3.1916866966 1 1 1<br>O 0.0000000000 0.0000000000 0.8826244595 1 1 1<br>K_POINTS automatic<br> 27 27 3 0 0 0<br>CELL_PARAMETERS alat<br> 0.999999999 -0.000000000 0.000000000 1 1 0<br> -0.500000000 0.866025404 0.000000000 1 1 0<br> -0.000000000 -0.000000000 6.705027151 0 0 0<br></div><div><b>SCF pw.x input file nearly identical minus the options for the relaxation calc. Also MXene's of this type are metallic so smearing is used. I use the same amount of smearing as the publication above.</b><br></div><div><br></div><b>%% Phonon ph.x input</b><br></div><div>&inputph<br> !recover=.true.,<br> tr2_ph=1.0d-12, ! desired conv_thr<br> ! tr2_ph=2.0d-9, ! used for dyn2 mode 1 !! pw_forum can ignore this<br> ! tr2_ph=2.0d-8, ! for the stubborn dyn2<br> max_seconds=324000,<br> prefix='ti3c2o2_phonons',<br> ldisp=.true.,<br> nq1=9, nq2=9, nq3=1,<br> start_q=2, last_q=2,<br> amass(1)=47.867,<br> amass(2)=12.01,<br> amass(3)=15.999,<br> outdir='./out_Ti3C2'<br> fildyn='ti3c2o2.dyn',<br>/</div><div><br></div><div><b>%% frequencies from gamma-only phonon calculation (input file not shown) but conv_thr was 1.0d-12</b><br></div><div> Diagonalizing the dynamical matrix<br><br> q = ( 0.000000000 0.000000000 0.000000000 )<br><br> **************************************************************************<br> freq ( 1) = 0.253345 [THz] = 8.450683 [cm-1]<br> freq ( 2) = 1.719726 [THz] = 57.363871 [cm-1]<br> freq ( 3) = 1.719726 [THz] = 57.363871 [cm-1]<br> freq ( 4) = 3.619841 [THz] = 120.744905 [cm-1]<br> freq ( 5) = 3.619841 [THz] = 120.744905 [cm-1]<br> freq ( 6) = 5.807810 [THz] = 193.727688 [cm-1]<br> freq ( 7) = 5.807810 [THz] = 193.727689 [cm-1]<br> freq ( 8) = 5.960133 [THz] = 198.808631 [cm-1]<br> freq ( 9) = 9.061633 [THz] = 302.263531 [cm-1]<br> freq ( 10) = 9.061633 [THz] = 302.263531 [cm-1]<br> freq ( 11) = 9.786433 [THz] = 326.440252 [cm-1]<br> freq ( 12) = 9.786433 [THz] = 326.440252 [cm-1]<br> freq ( 13) = 10.842274 [THz] = 361.659340 [cm-1]<br> freq ( 14) = 14.767962 [THz] = 492.606185 [cm-1]<br> freq ( 15) = 14.767962 [THz] = 492.606185 [cm-1]<br> freq ( 16) = 16.094128 [THz] = 536.842320 [cm-1]<br> freq ( 17) = 16.094128 [THz] = 536.842320 [cm-1]<br> freq ( 18) = 17.189295 [THz] = 573.373155 [cm-1]<br> freq ( 19) = 17.357952 [THz] = 578.998944 [cm-1]<br> freq ( 20) = 20.684968 [THz] = 689.976250 [cm-1]<br> freq ( 21) = 22.301687 [THz] = 743.904207 [cm-1]</div><div><br></div><div><b>%% Generated q-points for 9x9x1 MP grid</b></div><div> Dynamical matrices for ( 9, 9, 1) uniform grid of q-points<br> ( 12q-points):<br> N xq(1) xq(2) xq(3)<br> 1 0.000000000 0.000000000 0.000000000<br> 2 0.000000000 0.128304047 0.000000000<br> 3 0.000000000 0.256608094 0.000000000<br> 4 0.000000000 0.384912141 0.000000000<br> 5 0.000000000 0.513216187 0.000000000<br> 6 0.111114564 0.192456070 0.000000000<br> 7 0.111114564 0.320760117 0.000000000<br> 8 0.111114564 0.449064164 0.000000000<br> 9 0.111114564 0.577368211 0.000000000<br> 10 0.222229128 0.384912141 0.000000000<br> 11 0.222229128 0.513216187 0.000000000<br> 12 0.333343692 0.577368211 0.000000000</div><div><br></div><div><b>%% frequencies at Gamma-point from full dispersion calculation for conv_thr=1.0d-12</b><br></div><div> Diagonalizing the dynamical matrix<br><br> q = ( 0.000000000 0.000000000 0.000000000 )<br><br> **************************************************************************<br> freq ( 1) = -1.943043 [THz] = -64.812924 [cm-1]<br> freq ( 2) = -1.943043 [THz] = -64.812924 [cm-1]<br> freq ( 3) = -1.146697 [THz] = -38.249699 [cm-1]<br> freq ( 4) = 3.920804 [THz] = 130.783951 [cm-1]<br> freq ( 5) = 3.920804 [THz] = 130.783951 [cm-1]<br> freq ( 6) = 5.875260 [THz] = 195.977567 [cm-1]<br> freq ( 7) = 6.291059 [THz] = 209.847157 [cm-1]<br> freq ( 8) = 6.291059 [THz] = 209.847157 [cm-1]<br> freq ( 9) = 9.352154 [THz] = 311.954277 [cm-1]<br> freq ( 10) = 9.352154 [THz] = 311.954277 [cm-1]<br> freq ( 11) = 10.025397 [THz] = 334.411240 [cm-1]<br> freq ( 12) = 10.025397 [THz] = 334.411240 [cm-1]<br> freq ( 13) = 10.807507 [THz] = 360.499614 [cm-1]<br> freq ( 14) = 14.986167 [THz] = 499.884708 [cm-1]<br> freq ( 15) = 14.986167 [THz] = 499.884708 [cm-1]<br> freq ( 16) = 16.034628 [THz] = 534.857602 [cm-1]<br> freq ( 17) = 16.034628 [THz] = 534.857603 [cm-1]<br> freq ( 18) = 17.202121 [THz] = 573.801000 [cm-1]<br> freq ( 19) = 17.410826 [THz] = 580.762646 [cm-1]<br> freq ( 20) = 20.676124 [THz] = 689.681265 [cm-1]<br> freq ( 21) = 22.349113 [THz] = 745.486162 [cm-1]</div><div><br></div><div>(In my past simulations of various materials where I've achieved a good dispersion, I've noticed that small negative/imaginary frequencies near the gamma-point for acoustic modes are ok..)<br></div><div><br></div><b>%% frequencies at second q-point from full dispersion calculation after loosening conv_thr to 2.0d-8 due to convergence issues<br></b><div><div><div><b></b><div><div> Diagonalizing the dynamical matrix<br><br> q = ( 0.000000000 0.128304047 0.000000000 )<br><br> **************************************************************************<br> freq ( 1) = -58.972744 [THz] = -1967.119008 [cm-1]<br> freq ( 2) = -57.240460 [THz] = -1909.336227 [cm-1]<br> freq ( 3) = -48.504741 [THz] = -1617.944007 [cm-1]<br> freq ( 4) = -44.973984 [THz] = -1500.170631 [cm-1]<br> freq ( 5) = -24.011317 [THz] = -800.931308 [cm-1]<br> freq ( 6) = -20.216067 [THz] = -674.335394 [cm-1]<br> freq ( 7) = -14.459694 [THz] = -482.323488 [cm-1]<br> freq ( 8) = 5.896048 [THz] = 196.671001 [cm-1]<br> freq ( 9) = 10.557595 [THz] = 352.163448 [cm-1]<br> freq ( 10) = 12.651229 [THz] = 421.999584 [cm-1]<br> freq ( 11) = 15.876635 [THz] = 529.587523 [cm-1]<br> freq ( 12) = 19.799490 [THz] = 660.439907 [cm-1]<br> freq ( 13) = 21.309016 [THz] = 710.792280 [cm-1]<br> freq ( 14) = 24.971802 [THz] = 832.969663 [cm-1]<br> freq ( 15) = 25.083773 [THz] = 836.704597 [cm-1]<br> freq ( 16) = 25.193004 [THz] = 840.348172 [cm-1]<br> freq ( 17) = 32.881094 [THz] = 1096.795223 [cm-1]<br> freq ( 18) = 41.517549 [THz] = 1384.876376 [cm-1]<br> freq ( 19) = 68.062454 [THz] = 2270.319090 [cm-1]<br> freq ( 20) = 71.147302 [THz] = 2373.218533 [cm-1]<br> freq ( 21) = 72.136680 [THz] = 2406.220630 [cm-1]<br></div></div><div><br></div><div>I've bolded part of the forces output above that shows a large negative pressure in the z-direction. It is unclear to me if that would cause such behavior. <br><div><br></div><div>If anyone notices an error in my input files or has had some experience with MXene structures please feel free to chime in. Any advice is greatly appreciated.<br></div><div><br></div><div>Cheers,</div><div>Cameron<br></div><div>-- <br><div class="gmail_signature"><div dir="ltr"><div><div>NanoEnergy and Thermophysics Lab<br></div>Electrical and Computer Engineering Dept. <br></div>University of Massachusetts Amherst<br></div></div>
</div></div></div></div></div></div>