[QE-users] Why is my VC-RELAX not showing converged result?

[Lorenzo Paulatto]

hello,
vc-relax is done at constant fft grid, not constant kineti energy cutoff. At the end the scf is repeated with the initial cutoff, which can show some discrepancy if the calculation was not very converged, or if the initial volume was quite far from the final one. Repeat the vc-relax starting with the final cell and you should get a perfect result.

hth
--
Lorenzo Paulatto - Paris
On Feb 19 2022, at 5:58 pm, Naharin Jannath <naharin.jannath0o0 at gmail.com> wrote:
> I ran the VC-RELAX for simple niobium until I get a converged value. But at the bottom of the output, the cell parameter stops changing. Also, the pressure is well below the "pres_conv_thr" which was set to 0.01 in the input file. Can anyone tell me how to fix it? Also, my CELL_PARAMETER vs K curve is not smooth even using large smearing.
>
> &CONTROL
> calculation = 'vc-relax'
> restart_mode = 'from_scratch'
> prefix = 'Nb'
> etot_conv_thr = 0.000001
> pseudo_dir = '.'
> outdir = '.'
> disk_io = 'none'
> tstress = .true.
> tprnfor = .true.
> /
> &SYSTEM
> degauss = 0.05
> ecutrho = 360
> ecutwfc = 45
> ibrav = 0
> nat = 1
> ntyp = 1
> occupations = "smearing"
> smearing = "gaussian"
> /
> &ELECTRONS
> conv_thr = 1.0e-08
> electron_maxstep = 200
> mixing_mode = 'plain'
> mixing_beta = 0.5
> startingpot = "atomic"
> startingwfc = "atomic+random"
> /
> &IONS
> ion_dynamics = "bfgs"
> /
> &CELL
> cell_dynamics = "bfgs"
> press_conv_thr = 0.01
> /
> ATOMIC_SPECIES
> Nb 92.90638 Nb.pbe-spn-kjpaw_psl.0.3.0.UPF
> ATOMIC_POSITIONS {crystal}
> Nb 0.00 0.00 0.00
> CELL_PARAMETERS {angstrom}
> 1.675751271 1.675751271 1.675751271
> -1.675751271 1.675751271 1.675751271
> -1.675751271 -1.675751271 1.675751271
> K_POINTS {automatic}
> 10 10 10 0 0 0
>
>
> output --
> PRESSURE -
> total stress (Ry/bohr**3) (kbar) P= -47.02
> total stress (Ry/bohr**3) (kbar) P= -9.58
> total stress (Ry/bohr**3) (kbar) P= 0.65
> total stress (Ry/bohr**3) (kbar) P= 0.02
> total stress (Ry/bohr**3) (kbar) P= 0.01
> total stress (Ry/bohr**3) (kbar) P= -0.21
>
>
> CELL_PARAMETERS (angstrom)
> 1.662361144 1.662361144 1.662361144
> --
> CELL_PARAMETERS (angstrom)
> 1.659001159 1.659001159 1.659001159
> --
> CELL_PARAMETERS (angstrom)
> 1.659212618 1.659212618 1.659212618
> --
> CELL_PARAMETERS (angstrom)
> 1.659217666 1.659217666 1.659217666
> --
> CELL_PARAMETERS (angstrom)
> 1.659217666 1.659217666 1.659217666
>
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