Dear Anna If I remember well, the nearest-neighbor distance along z in the wurtzite crystals is not an inherent characteristic of its hexagonal symmetry, and should be therefore optimized also in a "relax" calculation. I suppose that when you shift from the hexagonal to the orthorombic supercell there are symmetry differences in the stress tensor that lead to differences (hopefully small, if the calculations are well-converged) in the c lattice parameters. HTH Giuseppe
Giuseppe Mattioli ISM-CNR Italy P.S. > ecutwfc = 80, > ecutrho = 700, probably too much for paw pseudopotentials. Didi yo find some instability of results at lower cutoffs? Quoting Anna Kuznetsova <[email protected]>: > Dear users, > > I am trying to achieve the perfect relaxation of hexagonal unit cell in > orthorhombic coordinates, however, I still see quite substantial residual > forces along z-dimension. Doing finer k-mesh sampling does not help with > the issue. > I am quite sure that the coordinates of atoms in each layer are correct, so > I was expecting that vc-relax will result only in the change in the unit > cell dimensions, but not the changes of atomic positions inside it. > > Please, give me some hints of what could be wrong in my input to help me > achieve full relaxation of the unit cell. > > > Input (also attached): > > &CONTROL > calculation = "vc-relax", > pseudo_dir = "/u/annakuz/espresso/pseudo", > outdir = "/lustre/work1/annakuz/tmp4", > / > &SYSTEM > ibrav = 8, > celldm(1) = 5.781869696282005, celldm(2) = 1.732050807568878, > celldm(3) = 1.632993161855452, > nat = 8, > ntyp = 2, > ecutwfc = 80, > ecutrho = 700, > occupations='smearing', smearing='gauss', degauss=0.03 > / > &ELECTRONS > conv_thr = 1.D-12 > / > &IONS > / > &CELL > / > ATOMIC_SPECIES > Si 28.0855 Si.pz-n-kjpaw_psl.0.1.UPF > C 12.0107 C.pz-n-kjpaw_psl.0.1.UPF > ATOMIC_POSITIONS {crystal} > Si 0.00 0.00 0.00 > C 0.00 0.00 0.375 > Si 0.50 0.50 0.00 > C 0.50 0.50 0.375 > Si 0.50 0.166666667 0.5 > C 0.50 0.166666667 0.875 > Si 1.00 0.666666667 0.5 > C 1.00 0.666666667 0.875 > > K_POINTS automatic > 15 8 9 1 1 1 > > > Output is attached but here is the final result of relaxation showing > forces still present in z-direction: > > ............ > convergence has been achieved in 31 iterations > Forces acting on atoms (Ry/au): > atom 1 type 1 force = 0.00000000 -0.00000037 0.00011491 > atom 2 type 2 force = 0.00000000 0.00000026 -0.00011491 > atom 3 type 1 force = 0.00000000 -0.00000037 0.00011493 > atom 4 type 2 force = 0.00000000 0.00000030 -0.00011491 > atom 5 type 1 force = 0.00000000 0.00000037 0.00011498 > atom 6 type 2 force = 0.00000000 -0.00000029 -0.00011498 > atom 7 type 1 force = 0.00000000 0.00000036 0.00011495 > atom 8 type 2 force = 0.00000000 -0.00000027 -0.00011497 > Total force = 0.000325 Total SCF correction = 0.000000 > > entering subroutine stress ... > total stress (Ry/bohr**3) (kbar) P= > -0.09 > -0.00000088 0.00000000 0.00000000 -0.13 0.00 0.00 > 0.00000000 -0.00000088 0.00000000 0.00 -0.13 0.00 > 0.00000000 0.00000000 0.00000001 0.00 0.00 0.00 > > bfgs converged in 3 scf cycles and 2 bfgs steps > (criteria: energy < 0.10E-03, force < 0.10E-02, cell < 0.50E+00) > End of BFGS Geometry Optimization > Final enthalpy = -250.0275444323 Ry > Begin final coordinates > new unit-cell volume = 547.64431 a.u.^3 ( 81.15251 Ang^3 ) > > CELL_PARAMETERS (alat= 5.78186970) > 0.998918439 0.000000000 0.000000000 > 0.000000000 1.730177232 0.000000000 > 0.000000000 0.000000000 1.639355189 > ATOMIC_POSITIONS (crystal) > Si 0.000000000 -0.000000091 -0.000421399 > C 0.000000000 -0.000000003 0.375421398 > Si 0.500000000 0.499999909 -0.000421402 > C 0.500000000 0.499999991 0.375421397 > Si 0.500000000 0.166666758 0.499578593 > C 0.500000000 0.166666675 0.875421409 > Si 1.000000000 0.666666758 0.499578597 > C 1.000000000 0.666666671 0.875421408 > End final coordinates > > Thank you for help! > -- > Best regards, > Anna -- ******************************************************** - Article premier - Les hommes naissent et demeurent libres et ègaux en droits. Les distinctions sociales ne peuvent être fondèes que sur l'utilitè commune - Article 2 - Le but de toute association politique est la conservation des droits naturels et imprescriptibles de l'homme. Ces droits sont la libertè, la propriètè, la sùretè et la rèsistance à l'oppression. ******************************************************** Giuseppe Mattioli CNR - ISTITUTO DI STRUTTURA DELLA MATERIA v. Salaria Km 29,300 - C.P. 10 I 00015 - Monterotondo Stazione (RM) Tel + 39 06 90672836 - Fax +39 06 90672316 E-mail: <[email protected]> _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
