Dear PWSCF users, I have a question regarding relaxation of low symmetry lattices. I did relaxation of 5 atoms FCC unit cell of copper. This is pure copper, no interface, no defect or anything. The unit cell is just not conventional, but a different way of choosing bulk FCC. For the default value of convergence the calculation finishes at the end of 1st scf cycle, but I see different forces when I describe nosym=.TRUE. or cell_parameters {cubic}. Following is the input file.
&control calculation='relax' pseudo_dir = '/home/manoj/espresso-4.0.4/pseudo', outdir='./', prefix='lcu', wf_collect=.TRUE. / &system ibrav =0, celldm(1)=6.81650937063832 nat= 5, ntyp= 1, ecutwfc =50.0, occupations='smearing', smearing='gaussian', degauss=0.02, ecutrho=500 / &electrons conv_thr = 1.0e-8 mixing_beta=0.7 / &ions ion_dynamics='bfgs' pot_extrapolation='second_order' wfc_extrapolation='second_order' / ATOMIC_SPECIES Cu 63.55 Cu.pz-d-rrkjus.UPF ATOMIC_POSITIONS Cu 0.0 0.0 0.0 Cu 0.5 0.5 0.0 Cu 0.0 1.0 0.0 Cu 1.0 1.0 0.0 Cu 0.5 1.5 0.0 K_POINTS (automatic) 8 8 8 0 0 0 CELL_PARAMETERS {cubic} 1.5 0.5 0.0 -0.5 1.5 0.0 0.0 0.5 0.5 Result of relaxation : The covergence is achieved with follwing forces on the atoms. Forces acting on atoms (Ry/au): Forces acting on atoms (Ry/au): atom 1 type 1 force = 0.00000000 0.00000000 0.00000000 atom 2 type 1 force = 0.00000567 0.00000185 -0.00000172 atom 3 type 1 force = 0.00001155 0.00000377 -0.00000372 atom 4 type 1 force = -0.00001155 -0.00000377 0.00000372 atom 5 type 1 force = -0.00000567 -0.00000185 0.00000172 I realized that I should not have chosen cubic symmetry group, as my unit cell is not cubic. So, as described in the input file parameters, for the low symmetry lattices, I specifed nosym=.TRUE., and again the relaxation stopped after 1 scf cycle, acheiving convergence. But the forces on the atoms are - Forces acting on atoms (Ry/au): atom 1 type 1 force = 0.00001355 0.00000524 -0.00000516 atom 2 type 1 force = -0.00001493 -0.00000395 0.00000395 atom 3 type 1 force = 0.00000253 -0.00000043 0.00000045 atom 4 type 1 force = 0.00000820 0.00000230 -0.00000239 atom 5 type 1 force = -0.00000935 -0.00000316 0.00000315 So, still forces are .001 ev/a.u, which i think is higher for a perfect FCC lattice. I can specify stricter convergence of force, but that would move atoms around and break symmetry. So, as long as the lattice parameter is correct, I think for perfect FCC lattice, forces on atoms should be smaller than what they are presently. I have checked the lattice constant for bulk FCC with a different calculation, so lattice constant is not an issue. Another possibility is symmetry group, as you can see with the same input variables, changing symmetry group changes results.I was wondering if there is a way i can choose correct symmetry group for this low symmetry lattice which might give me smaller forces? Are there any other parameters I need to specify? On a broad view I am curious to know how to do relaxation of low symmetry lattices. Regards, Manoj Srivastava University of Florida Gainesville, USA