Dear all, while learning how QE works, i'm trying to relax ion positions at the [100] surface of FCC Aluminum.
Looking at Exercise 4 of day1, QE workshop (2012), i made an input file to relax ion position (starting from ideal) while keeping 2 layers fixed in the "bulk" (asymmetric slab model). I consider 5 unit cells + 3 vacuum in X direction (10 layers) and 1 unit cell in Y/Z. Results of layers displacements (Bohr) as well as change in interlayer distance relative to the perfect distance (7.5 / 2 Bohr ) in % are as follows: 0 -0.015027115 -0.00602681546667 1 0.007573443 -0.0030597528 2 0.019047516 0.0012871304 3 0.014220777 0.002283688 4 0.005656947 0.00307896053333 5 -0.005889155 0.00220971653333 6 -0.014175592 0.000569633599999 7 -0.016311718 -0.004349796 8 1.69999978539e-08 -5.33333377462e-10 9 1.89999980194e-08 This looks wrong to me. To have some reference I did the same calculation in LAMMPS using EAM potential and atom displacements in X are in range of -0.190234 to -0.0226957 Angstroms, monotonously decreasing towards the fixed layers, as i would expect it to be. It seems something is wrong with DFT calculations (relaxation) i did. The input script is below. If anyone can spot something suspicious, please let me know. I also did a convergence study on Al.pz-vbc.UPF potential for bulk crystal. So it pretty much converged in terms of total energy for ecutwfc, degauss and number of K points i used. Kind regards, Denis ---- pw.x input file ---- &CONTROL calculation = "relax", prefix = 'Al_lammps' / &SYSTEM ibrav = 8, celldm(1) = 60, celldm(2) = 0.125, celldm(3) = 0.125, nat = 20, ntyp = 1, ecutwfc = 35, occupations = "smearing", smearing = "marzari-vanderbilt", degauss = 0.005, / &ELECTRONS conv_thr = 1e-07, mixing_beta = 0.7, / &IONS bfgs_ndim = 3, / ATOMIC_SPECIES Al 1.0 Al.pz-vbc.UPF ATOMIC_POSITIONS bohr Al 22.5 1.875 1.875 1 1 1 Al 26.25 5.625 1.875 1 1 1 Al 26.25 1.875 5.625 1 1 1 Al 22.5 5.625 5.625 1 1 1 Al 30 1.875 1.875 1 1 1 Al 33.75 5.625 1.875 1 1 1 Al 33.75 1.875 5.625 1 1 1 Al 30 5.625 5.625 1 1 1 Al 37.5 1.875 1.875 1 1 1 Al 41.25 5.625 1.875 1 1 1 Al 41.25 1.875 5.625 1 1 1 Al 37.5 5.625 5.625 1 1 1 Al 45 1.875 1.875 1 1 1 Al 48.75 5.625 1.875 1 1 1 Al 48.75 1.875 5.625 1 1 1 Al 45 5.625 5.625 1 1 1 Al 52.5 1.875 1.875 0 0 0 Al 56.25 5.625 1.875 0 0 0 Al 56.25 1.875 5.625 0 0 0 Al 52.5 5.625 5.625 0 0 0 K_POINTS (automatic) 4 16 16 1 1 1
