[Pw_forum] Ionic forces not reducing

[Gabriele Sclauzero]

Il giorno 24/feb/2011, alle ore 10.10, giuseppe.mattioli at mlib.ism.cnr.it ha scritto:

> 
> 
> 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).

Alternatively you can use upscale=1.D3 in &IONS (the default should be 1.D1, if I remember well).


> 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.

This threshold looks very tight to me. The default of 1.D-3 is often sufficient. If you really need such a good accuracy, you might also need to increase the cutoffs to get more precise forces. All the elements in your compound are pretty unfriendly to planewaves, even when using ultrasoft pseudos (Cu and Ti have 3d electrons in valence, O has 2p).
I suggest you to perform a test on the force convergence w.r.t. ecutwfc.

HTH

GS


>> 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
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> 
> Giuseppe Mattioli
> ISM-CNR, Italy
> 
> 
> 
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§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne

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