Molecular crystal in a triclinic cell

Alex nedo... at gmail.com
Wed Oct 7 20:34:50 CEST 2015


Thanks Matt.

I enforced the angles and changed my SCF section to, hope this looks 
reasonable:

    &SCF
      &OT ON
       MINIMIZER DIIS
       PRECONDITIONER FULL_ALL
       ENERGY_GAP 0.001
      &END OT
      SCF_GUESS RESTART 
      EPS_SCF 5.0E-6
      MAX_SCF 300
    &END SCF

Given the above, I do not see much of an improvement speed-wise, to be 
honest. The memory footprint is about half of what it was, I can see that 
much.

Your comment regarding XC is definitely well taken, we here don't believe 
PBE+VDW out of the box should be accurate. I am tempted to try the setup 
from p. 21 of the pdf you suggested, but does this BEEF work in stock CP2K 
and does one need to change the potentials (I am using GTH potentials). I 
suppose this boils down to whether there is an input file from the authors 
for others to try...

Alex


On Wednesday, October 7, 2015 at 2:37:42 AM UTC-6, Matt W wrote:
>
> Hi Alex,
>
> one concern would be the XC functional. There are some slides from a 
> recent CECAM meeting at 
> http://www.cp2k.org/_media/events:2015_cecam_tutorial:ling_vdw.pdf with 
> some partial inputs of setting up DFT-vdw functionals. I'm not sure PBE+vdw 
> is great.
>
> Otherwise, there are some things that might be a bit more efficient:
>
> Changing to the OT method instead of using diagonalization would give you 
> a very large speed up, I would think (order of magnitude probably here).
>
> It might be a good idea to fix the angles of the cell and do an 
> optimization (CELL_OPT/KEEP_ANGLES) then optimize again without the 
> constraint.
>
> Matt
>
> On Tuesday, October 6, 2015 at 7:35:18 PM UTC+1, Alex wrote:
>>
>> Hi all,
>>
>> We're trying to simulate an n-octane crystal (triclinic, coordinates and 
>> cell vectors from literature) -- just a static cell + geometry 
>> optimization. Here's the issue: when simulating a single cell (1 1 1), the 
>> crystal cohesive energy is far above the experiment -- an order of 
>> magnitude larger in absolute value. As a vacuum ref we're simulating an 
>> isolated octane molecule. In all cases, a NON_LOCAL vdw energy correction 
>> is used.
>>
>> When simulating a total of eight cells (2 2 2), and the simulation is 
>> still running, so the results are preliminary, things appear to be much 
>> more reasonable. The input file is below (I can provide the coordinates as 
>> well, if needed). Can you just by simple inspection tell me if anything is 
>> obviously wrong?
>>
>> Thanks a lot,
>>
>> Alex
>>
>> **********************************
>> &GLOBAL
>>   PROJECT opt
>>   PRINT_LEVEL MEDIUM 
>>   RUN_TYPE CELL_OPT
>> &END GLOBAL
>>  &MOTION
>>   &GEO_OPT
>>     OPTIMIZER BFGS 
>>   &END
>>   &CELL_OPT
>>     EXTERNAL_PRESSURE [bar] 1.0
>>     OPTIMIZER BFGS
>>   &END
>> &END MOTION
>>  
>> &FORCE_EVAL
>>   METHOD QS
>>   STRESS_TENSOR ANALYTICAL
>>   &DFT
>>     BASIS_SET_FILE_NAME BASIS_SET2
>>     POTENTIAL_FILE_NAME GTH_POTENTIALS2
>>     &MGRID
>>       NGRIDS 5
>>       CUTOFF 500
>>     &END MGRID
>>     &QS
>>       METHOD GPW
>>     &END QS
>>     &SCF
>>       SCF_GUESS RESTART 
>>       EPS_SCF 5.0E-6
>>       MAX_SCF 300
>>       ADDED_MOS  100 
>>       &MIXING
>>           METHOD BROYDEN_MIXING
>>           ALPHA    0.1
>>           NBROYDEN   8
>>       &END MIXING
>>     &END SCF
>>     &XC
>>       &XC_FUNCTIONAL PBE
>>       &END XC_FUNCTIONAL
>>       &VDW_POTENTIAL NON_LOCAL
>>       &END VDW_POTENTIAL
>>     &END XC
>>   &END DFT
>>   &SUBSYS
>>     &CELL
>>   ABC 4.160 4.750 11.000
>> ANGLES 94.80  84.50 105.10
>>         PERIODIC XYZ
>> SYMMETRY TRICLINIC
>>         MULTIPLE_UNIT_CELL 2 2 2 
>>     &END
>>     &TOPOLOGY
>>        COORD_FILE_NAME noct_2mol.pdb
>>        COORD_FILE_FORMAT PDB
>>        MULTIPLE_UNIT_CELL  2 2 2 
>>     &END
>>     &KIND C
>>       BASIS_SET  DZVP-MOLOPT-SR-GTH
>>       POTENTIAL  GTH-PBE-q4
>>     &END KIND
>>     &KIND H
>>       BASIS_SET  DZVP-MOLOPT-SR-GTH
>>       POTENTIAL  GTH-PBE-q1
>>     &END KIND
>> &END SUBSYS
>> &END
>>
>>
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