Molecular crystal in a triclinic cell

[Matt W]

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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