<div>Dear PaoloU,</div> <div> </div> <div>Thanks for your help. I now still have some puzzlements about the procedure of </div> <div>calculating vibrationl spectrum by finite electric field method.</div> <div> </div> <div>1.I consider a model structure of tetrahedral amorphous carbon obtained by my schoolmate using a Car-Parrinello molecular dynamics approach. </div> <div> </div> <div>2.When running a self-consistent calculation for this structure (using different exchange-crrelation energy and pseudopotential with in CPMD), I found the forces and stress are big.<BR>In this case, I should use pw.x to compute the equilibrium geometry <BR>before running finite electric field method to calculate Raman tensor. Is it right?<BR> <BR> Total force = 0.633437 Total SCF correction = 0.000080</div> <div> total stress
(Ry/bohr**3) (kbar) P= -69.10<BR> -0.00087420 0.00095392 -0.00017791 -128.60 140.33 -26.17<BR> 0.00095392 0.00033110 0.00005680 140.33 48.71 8.35<BR> -0.00017791 0.00005680 -0.00086609 -26.17 8.35 -127.41</div> <div> </div> <div>3.I run a vc-relax calculation and input and output files are as follows.<BR>It is obvious the cell angle parameters have been changed and the structure isn't the cubic structure. </div> <div> </div> <div>(1)ATOMIC_POSITIONS (angstrom)<BR>&CONTROL<BR>
calculation = "vc-relax",<BR> prefix = "ta-c",<BR> pseudo_dir = "/home/niuli/espresso-3.2/pseudo",<BR> outdir = "/home/niuli/tmp",<BR> etot_conv_thr=1.0D-4,<BR> forc_conv_thr=1.0D-5<BR> tstress = .true. ,<BR> tprnfor = .true. ,<BR> nstep = 65 ,<BR> dt = 50 ,<BR>/<BR>&SYSTEM<BR> ibrav = 1, <BR> celldm(1) = 13.9103, <BR> nat= 64, ntyp= 1,<BR> ecutwfc =55 <BR>/<BR>&ELECTRONS<BR> conv_thr = 1.D-9,<BR> mixing_beta = 0.7D0,<BR>/<BR>&IONS<BR>
<BR>/<BR>&CELL<BR> cell_dynamics = 'damp-w' ,<BR> press = 0.0 ,<BR>/<BR>ATOMIC_SPECIES<BR> C 12.0107 C.pz-mt.UPF</div> <div>ATOMIC_POSITIONS (angstrom)<BR> C 0.335930 -0.353760 -0.911160<BR> C 1.410315 -0.756669 3.705328<BR> C -0.462432 3.049465 -0.168292<BR> ...<BR>K_POINTS automatic<BR>1 1 1 0 0 0<BR>----------------------------------------------------------<BR>final unit-cell volume = 2649.6086 (a.u.)^3<BR> input alat = 13.9103
(a.u.)</div> <div>CELL_PARAMETERS (alat)<BR> 0.988103212 0.021498496 -0.002655708<BR> 0.021493430 1.013103454 -0.000556700<BR> -0.002655491 -0.000556401 0.983829644</div> <div>ATOMIC_POSITIONS (angstrom)<BR>C 0.292559767 -0.288781798 -0.891844186<BR>C 1.355532741 -0.743068572 3.628414295<BR>C -0.355484435 3.081913445 -0.167170533<BR>C 0.095087835 3.109571795 3.835737135<BR>C 4.156057825 0.859970231 -0.191362362<BR>...<BR>-------------------------------------------------------------<BR>(2)ATOMIC_POSITIONS (crystal)</div> <div><BR>CELL_PARAMETERS (alat)<BR> 0.997839246
0.002690608 -0.000417664<BR> 0.002690608 1.001016967 0.000136231<BR> -0.000417664 0.000136231 0.997967842<BR> <BR>ATOMIC_POSITIONS (crystal)<BR>C 0.040549747 -0.039126226 -0.123246079<BR>C 0.194302645 -0.104600063 0.504317492<BR>C -0.056833511 0.414314558 -0.024646663<BR>C 0.005446848 0.417400176 0.531096601<BR>C 0.569734741 0.101952019 -0.026079258<BR>C 0.849788292 0.163379218 0.703668015<BR>C 0.572774545 0.583287732
0.029202254<BR>...<BR>------------------------------------------------------------</div> <div>4.If I set ibrav=0 and use the obtained new cell parameter and atomic positions in the next scf calculation, the results on forces and stress are the same with that in vc-relax.<BR>But if I set ibrav=1 in the next scf calculation, how to read the new lattice constant?<BR> <BR>5. Then run ph.x code to obtain the phonon frequencies and eigenmodes. The phonon density of states is also obtained by dynmat.x code.<BR>In your papers,the dynamical matrix is calculated by taking finite differences of atomic forces for atomic displacements of a constant bohr.What is the difference between two methods?<BR> Why don't you use the linear response method(ph.x code) to calculate the vibrational properties? </div> <div> </div> <div>6. Supposing I obtain the equilibrium geometry of ibrav= 1, I then can calculate Raman tensor by considering second derivatives of the
atomic forces(cp.x code).<BR> <BR>Is my thought right on the procedure of calculating?<BR> <BR>Best wishes,</div> <div> </div> <div>Niu Li<BR>Harbin Institute of Technology<BR>China<BR></div><p>
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