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    <div class="moz-cite-prefix">   Dear Suresh,<br>
      <br>
         Variable-cell-shape geometry optimization is quite stable in QE
      and <br>
         free from human errors caused by ignorance or wrong
      implementation <br>
         of some necessary steps in hand calculations which takes our
      valuable time.  <br>
         Beside in QE you need not worry about symmetry conservation
      (unlike SIESTA : <br>
         in siesta you should write your own routine to massage
      strains). A year ago I was <br>
         trying to determine lattice parameters of \alpha-Uranium  and
      qe's accuracy <br>
         was in line between experimental and full-potential values. <br>
         <br>
         Note that initial guess for lattice paramaters is important but
      even for bad <br>
         guess (some degree, of course!) their code is smart enough on
      choosing <br>
         right direction within optimization process.<br>
      <br>
         Savaş AĞDUK<br>
         Research Assistant<br>
         Karabuk University<br>
         Physics Department<br>
      <br>
      On 11/06/2013 01:29 PM, Suresh A wrote:<br>
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cite="mid:CAE_UR+WL__v1AgvZ3cXG8HmkWUG=-Xv6gcx8zmCyb0SkaQAKVg@mail.gmail.com"
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                  <div>I am new to QE. I have done variable cell volume
                    optimization for rutile tio2.<br>
                  </div>
                  I confused with finding ground state structure. <br>
                  <br>
                </div>
                In literature survey the authors did volume- constraint
                total energy minimization and have set cell parameters
                a, c and internal parameter(  u for rutile)<br>
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              finally fitted their value Birch - Murnaghan equation(B-M)
              . From the final value they have compared their values
              with exprimental values of cell parameter <br>
              a,c and elastic parameters.<br>
              <br>
            </div>
            my question is:<br>
            <br>
          </div>
           i) which method is correct whether doing doing vc-relax
          alone  or collect all volume and total energy from vc-relax
          output and fitting this with B-M  equation ?<br>
          <br>
        </div>
         ii) How to choose internal parameter u for TiO2 rutile.?<br>
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                  <div>i have included the input file. <br>
                    <br>
                     &CONTROL<br>
                                           title = 'rutile' ,<br>
                                     calculation = 'vc-relax' ,<br>
                                    restart_mode = 'from_scratch' ,<br>
                                          outdir =
                    '/home/basheer/Desktop/relax/' ,<br>
                                      pseudo_dir =
                    '/home/basheer/espresso-5.0.1/pseudo/' ,<br>
                                          prefix = 'rutile' ,<br>
                                   etot_conv_thr = 1.d-6 ,<br>
                                   forc_conv_thr = 1.d-4 ,<br>
                                         tstress = .true. ,<br>
                                         tprnfor = .true. ,<br>
                     /<br>
                     &SYSTEM<br>
                                           ibrav = 6,<br>
                                               A = 4.59373 ,<br>
                                               B = 4.59373 ,<br>
                                               C = 2.95812 ,<br>
                                           cosAB = 0 ,<br>
                                           cosAC = 0 ,<br>
                                           cosBC = 0 ,<br>
                                             nat = 6,<br>
                                            ntyp = 2,<br>
                                         ecutwfc = 200 ,<br>
                                         ecutrho = 800 ,<br>
                                     occupations = 'smearing' ,<br>
                                         degauss = 0.01 ,<br>
                                        smearing = 'gaussian' ,<br>
                                exxdiv_treatment = 'gygi-baldereschi' ,<br>
                     /<br>
                     &ELECTRONS<br>
                                        conv_thr = 1.d-12 ,<br>
                     /<br>
                     &IONS<br>
                     /<br>
                     &CELL<br>
                                   cell_dynamics = 'bfgs' ,<br>
                                  press_conv_thr = 2 ,<br>
                     /<br>
                    ATOMIC_SPECIES<br>
                       Ti   47.88800  Ti.pz-mt_fhi.UPF <br>
                        O   15.99900  O.pz-mt_fhi.UPF <br>
                    ATOMIC_POSITIONS angstrom <br>
                       Ti      0.000000000    0.000000000   
                    0.000000000    <br>
                       Ti      2.296860000    2.296860000   
                    1.479060000    <br>
                        O      1.402470000    1.402470000   
                    0.000000000    <br>
                        O      3.191260000    3.191260000   
                    0.000000000    <br>
                        O      0.894400000    3.699330000   
                    1.479060000    <br>
                        O      3.699330000    0.894400000   
                    1.479060000    <br>
                    K_POINTS automatic <br>
                      4 4 4   1 1 1 <br>
                    <br>
                    <br clear="all">
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                                  <div style="text-align:right">A.Suresh,<br>
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                                <div style="text-align:right">Project
                                  Fellow,<br>
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                              <div style="text-align:right">Madurai
                                Kamaraj University,<br>
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                              Madurai.<br>
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