Fixing a bond in DFT

[Andreas Funk]

Dear Wei,

The first thing I see in your input is that you specify one colvar in 
SUBSYS, but in MOTION you call two colvars. This cannot work. As it is only 
one bond you specify as colvar, you should work with one colvar in the 
CONSTRAINT section. Then you should keep it "fixed" like this in section 
MOTION:

&CONSTRAINT
  &COLLECTIVE
    COLVAR 1
    INTERMOLECULAR
    TARGET 2.399706
    &RESTRAINT
      K 1.0
    &END RESTRAINT
  &END COLLECTIVE
&END CONSTRAINT

Please keep in mind that the distance you want it to be fixed (TARGET) is 
in bohrs, not angstroms. You need to experiment a little with the force 
constant for the restraint. Check it in your runs if it is sufficient to 
keep the bond length fixed within your desired accuracy. In principle, it 
should do the job. I have used this kind of constraint successfully for 
both, geometry optimizations and MD.

Best regards

Andreas


Am Donnerstag, 13. November 2014 12:31:24 UTC+1 schrieb Wei:
>
> Dear Pankaj, Ari and Marco,
>
> I'd also like to fix a bond during a MD (DFT) run (not the absolute atomic 
> positions). I tried to specify CP2K_INPUT / MOTION / CONSTRAINT / 
> COLLECTIVE and CP2K_INPUT / FORCE_EVAL / SUBSYS / COLVAR by following an 
> expample from regtest /Fist/regtest-12/water_3_dist_2.inp.
>
> However, the calculation always crashes with an error:
>
>  *
>  *** 12:16:30 ERRORL2 in topology_input:read_constraints_section 
> processor  ***
>  ***      0  err=-300  condition FAILED at line 
> 561                         ***
>  *
>
>  ===== Routine Calling Stack =====
>
>             3 topology_control
>             2 quickstep_create_force_env
>             1 CP2K
>  CP2K| condition FAILED at line 561
>  CP2K| Abnormal program termination, stopped by process number 0
>
>
> Could you please give me some hint on this issue? I put my input file 
> below. Thanks in advance!
>
> Best Regards
>
> Wei
>
> input 
>
> &FORCE_EVAL
>   METHOD QUICKSTEP
>   &DFT
>     BASIS_SET_FILE_NAME ./GST.qbs
>     POTENTIAL_FILE_NAME ./GTH_POTENTIALS
>     &MGRID
>       CUTOFF 300
>     &END MGRID
>     &QS
>       EPS_DEFAULT 1.0e-12
>       EXTRAPOLATION ASPC
>       EXTRAPOLATION_ORDER 0
>       MAP_CONSISTENT
>     &END QS
>     &SCF
>       EPS_SCF 1.0e-7
>       MAX_SCF 40
>       MAX_SCF_HIST 2
>       &OT ON
>         MINIMIZER DIIS
>         PRECONDITIONER FULL_KINETIC
>         ENERGY_GAP 0.002
>         STEPSIZE 0.0875
>       &END OT
>       &OUTER_SCF ON
>         MAX_SCF 800
>         EPS_SCF 1.0e-7
>       &END OUTER_SCF
>       SCF_GUESS RANDOM
> #     SCF_GUESS HISTORY_RESTART
>     &END SCF
>     &XC
>       &XC_FUNCTIONAL PBE
>       &END XC_FUNCTIONAL
>     &END XC
>   &END DFT
>   &SUBSYS
>     &CELL
>       ABC   18.62463  18.62463  18.62463
>     &END CELL
>     &COORD
>  Ge 10.0 10.0 10.0  bond
>  Te 13.0 10.0 10.0  bond
>     &END COORD
>     &COLVAR
>       &DISTANCE
>         ATOMS 1 2
>       &END DISTANCE
>     &END COLVAR
>     &KIND Ge
>       BASIS_SET DZVP-MOLOPT-SR-GTH-q4
>       POTENTIAL GTH-PBE-q4
>     &END KIND
>     &KIND Te
>       BASIS_SET DZVP-MOLOPT-SR-GTH-q6
>       POTENTIAL GTH-PBE-q6
>     &END KIND
>   &END SUBSYS
> &END FORCE_EVAL
> &GLOBAL
>   PREFERRED_FFT_LIBRARY FFTW
>   PRINT_LEVEL LOW
>   PROJECT GeTe.md
>   RUN_TYPE MD
> &END GLOBAL
>
> &MOTION
>   &MD
>     ENSEMBLE LANGEVIN
>     STEPS    20
>     TEMPERATURE  1100
>     TIMESTEP 2.0
>     &LANGEVIN
>       GAMMA 4.0E-3
>       NOISY_GAMMA 3.5E-5
>     &END LANGEVIN
>   &END MD
>   &CONSTRAINT
>     &COLLECTIVE
>       MOLNAME  bond
>       COLVAR 1
>     &END COLLECTIVE
>     &COLLECTIVE
>       MOLNAME  bond
>       COLVAR 2
>     &END COLLECTIVE
>   &END CONSTRAINT
> &END MOTION
>
>>
>>>
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