[Pw_forum] Ionic forces not reducing

[Christopher O'Brien]

Thank you for your suggestions regarding my problem. In my case, a further decrease in the electronic convergence threshold, or increase in ecutrho, and ecutwfc had little effect. I usually work with supercells or NEB calculations with little or no symmetry.
It turns out that the parameter that affected the most significant reduction in force was the "upscale" parameter. Your recommended value of 1.D3 was usually sufficient, but really difficult cases can be handled with 1.D4. This parameter seemed to be the 'magic bullet' for me when nothing else worked. Why is the default value so small (10.0)?
===================================================================
Christopher J. O'Brien
cjobrien at ncsu.edu
https://sites.google.com/a/ncsu.edu/cjobrien/

Ph.D. Candidate
Computational Materials Group
Department of Materials Science & Engineering
North Carolina State University
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Please send all documents in PDF. 
For Word documents: Please use the 'Save as PDF' option before sending.
===================================================================

On Feb 24, 2011, at 4:08 AM, Giuseppe Mattioli wrote:

> Dear Christopher
> You should try to reduce conv_thr in &ELECTRONS, in order to have a more accurate calculation of forces and stress. Try, for instance, with conv_thr = 1.0d-9 (and with a lower mixing_beta, 0.2~0.3 or so, if the electronic calculation does not converge).
> HTH
> 
> Giuseppe
> 
> Quoting Christopher O'Brien <cjobrien at ncsu.edu>:
> 
>> Below this is a super-cell of an interface of Cu2O and Ti. The  problem is simply that the forc_conv_thr is not met within 100  iterations. etot_conv_thr is almost always met first (even  decreasing etot_conv_thr to 1.0d-6). Typically, the force in this  system decreases to ~0.01 Ry/B with a delta F on the same order of  magnitude but fluctuates about this value. This means that I  continually get the warning telling me to reduce the convergence  threshold. Ideally I would like obtain a maximum force of ~1D-4.
>> Does anyone have any suggestions? I am at a loss.
>> 
>> Thanks in advance,
>> Chris O'Brien
>> 
>> 
>> title = 'Cu2O-Ti-case2' ,
>> calculation = 'vc-relax' ,
>> restart_mode = 'from_scratch' ,
>> wf_collect = .false. ,
>> outdir = './',
>> wfcdir = './',
>> pseudo_dir = '/share2/cjobrien/pseudo/',
>> prefix = 'Cu2OTi',
>> disk_io = 'low',
>> verbosity = 'minimal',
>> tstress = .true.,
>> tprnfor = .true.,
>> forc_conv_thr = 1.0d-4,
>> etot_conv_thr = 1.0d-5,
>> /
>> &SYSTEM
>> ibrav = 0,
>> celldm(1) = 8.1878,
>> nat = 14,
>> ntyp = 3,
>> ecutwfc = 30.0 ,
>> ecutrho = 300.0 ,
>> occupations = 'smearing' ,
>> smearing = 'mv' ,
>> degauss = 0.01 ,
>> /
>> &ELECTRONS
>> conv_thr = 1.0d-6 ,
>> mixing_beta = 0.7 ,
>> /
>> &IONS
>> ion_dynamics = 'bfgs',
>> /
>> &CELL
>> cell_dynamics = 'bfgs',
>> press = 0.0,
>> cell_dofree = 'xyz',
>> /
>> CELL_PARAMETERS
>> 0.929429709 0.0 0.0
>> 0.0 0.929429709 0.0
>> 0.0 0.0 2.924945006
>> ATOMIC_SPECIES
>> Cu   58.70000  Cu.pw91-n-van_ak.UPF
>> Ti   47.88000  Ti.pw91-nsp-van.UPF
>> O    16.00000  O.pw91-van_ak.UPF
>> ATOMIC_POSITIONS alat
>> Cu      0       0       0
>> Cu      0.504691013     0.504691013     0
>> Cu      0.484702934     0.019988079     0.553031754
>> Cu      0.019988079     0.484702934     0.553031754
>> Cu      0.039976158     0.039976158     1.106063508
>> Cu      0.464714854     0.464714854     1.106063508
>> O       0.252345506     0.252345506     0.285738983
>> O       0.024221288     0.024221288     1.544049139
>> Ti      0.480469725     0.024221288     1.544049139
>> Ti      0.024221288     0.480469725     1.544049139
>> Ti      0.484702934     0.484702934     2.015504257
>> Ti      0.019988079     0.019988079     2.015504257
>> Ti      0.01575487      0.488936142     2.486959376
>> Ti      0.488936142     0.01575487      2.486959376
>> K_POINTS automatic
>> 8 8 4 0 0 0
>> 
>> ===================================================================
>> Christopher J. O'Brien
>> cjobrien at ncsu.edu
>> https://sites.google.com/a/ncsu.edu/cjobrien/
>> 
>> Ph.D. Candidate
>> Computational Materials Group
>> Department of Materials Science & Engineering
>> North Carolina State University
>> __________________________________________________________________
>> Please send all documents in PDF.
>> For Word documents: Please use the 'Save as PDF' option before sending.
>> ===================================================================
>> 
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>> 
> 
> Giuseppe Mattioli
> ISM-CNR, Italy
> 
> 
> 
>