[CP2K-user] QM/MM energy conservation

[may...@gmail.com]

Hi,

I am trying a NVT MD simulation for a QM/MM system with QM water molecules 
surrounding a spatially fixed MM molecule, which are in turn surrounded by 
MM water molecules. I am using a time step of 5 a.u. (for particular 
reasons) and the QM box is much smaller than compared to full MM box so 
within the timescale of MD simulation the QM molecules do not cross the QM 
box boundary. I am attaching the relevant sections of input file:
----
 &MOTION
   &MD
     ENSEMBLE  NVT
     STEPS  10000
     TIMESTEP     1.21E-01
     TEMPERATURE     3.00E+02
     &THERMOSTAT
       TYPE  NOSE
       REGION  DEFINED
       &DEFINE_REGION
         QM_SUBSYS  ATOMIC
       &END DEFINE_REGION
       &DEFINE_REGION
         MM_SUBSYS  ATOMIC
       &END DEFINE_REGION
       &NOSE
         TIMECON     9.99E+02
       &END NOSE
     &END THERMOSTAT

 &FORCE_EVAL
   METHOD  QMMM
   &DFT
     BASIS_SET_FILE_NAME ./GTH_BASIS_SETS
     POTENTIAL_FILE_NAME ./GTH_POTENTIALS
     CHARGE  0
     &SCF
       MAX_SCF  60
       EPS_SCF     2.0E-07
       SCF_GUESS  RESTART
       &OT  T
         MINIMIZER  DIIS
         PRECONDITIONER  FULL_SINGLE_INVERSE
       &END OT
       &OUTER_SCF  T
         EPS_SCF     2.0E-07
         MAX_SCF  40
       &END OUTER_SCF
       &PRINT
         &RESTART  SILENT
           BACKUP_COPIES  1
         &END RESTART
       &END PRINT
     &END SCF
     &QS
       EXTRAPOLATION  ASPC
       EXTRAPOLATION_ORDER  4
     &END QS
     &MGRID
       CUTOFF     3.2E+02
       COMMENSURATE  T
     &END MGRID
     &XC
       DENSITY_CUTOFF     1.0E-10
       GRADIENT_CUTOFF     1.0E-10
       TAU_CUTOFF     1.0E-10
       &XC_GRID
         XC_SMOOTH_RHO  NN50
         XC_DERIV  SPLINE2_SMOOTH
       &END XC_GRID
       &XC_FUNCTIONAL  NO_SHORTCUT
         &PBE  T
           PARAMETRIZATION  REVPBE
         &END PBE
       &END XC_FUNCTIONAL
       &VDW_POTENTIAL
         POTENTIAL_TYPE  PAIR_POTENTIAL
         &PAIR_POTENTIAL
           R_CUTOFF     1.9E+01
           TYPE  DFTD3
           PARAMETER_FILE_NAME ./dftd3.dat
           REFERENCE_FUNCTIONAL revPBE
           CALCULATE_C9_TERM  T
           REFERENCE_C9_TERM  T
           LONG_RANGE_CORRECTION  F
         &END PAIR_POTENTIAL
       &END VDW_POTENTIAL
     &END XC
   &END DFT
   &MM
     &FORCEFIELD
       ...
     &END FORCEFIELD
     &POISSON
       &EWALD
         EWALD_TYPE  SPME
         RCUT     1.2E+01
         ALPHA     2.917E-01
         GMAX  75
         O_SPLINE  4
       &END EWALD
     &END POISSON
   &END MM
   &QMMM
     E_COUPL  GAUSS
     USE_GEEP_LIB  6
     NOCOMPATIBILITY  T
     CENTER  NEVER
     INITIAL_TRANSLATION_VECTOR     0.0E+00    0.0E+00    0.0E+00
     &QM_KIND O
       MM_INDEX  ...
     &END QM_KIND
     &QM_KIND H
       MM_INDEX  ...
     &END QM_KIND
     &MM_KIND H
       RADIUS     4.4E-01
     &END MM_KIND
     &MM_KIND O
       RADIUS     1.2E+00
     &END MM_KIND
     .....
     &CELL
       ABC     4.0E+01    3.0E+01    3.0E+01
       PERIODIC  XYZ
     &END CELL
     &PERIODIC
       GMAX     5.0E-01
       &MULTIPOLE  ON
         RCUT     1.2E+01
         EWALD_PRECISION     1.0E-08
         ANALYTICAL_GTERM  T
       &END MULTIPOLE
     &END PERIODIC
   &END QMMM
   &SUBSYS
     &CELL
       A     5.0E+01    0.0E+00    0.0E+00
       B     0.0E+00    4.0E+01    0.0E+00      
       C     0.0E+00    0.0E+00    4.0E+01
       MULTIPLE_UNIT_CELL  1 1 1
     &END CELL
     &COORD

-------

With this setup for the production run I obtained the following trends for 
the potential and constant quantity as the graphs attached here. I have 
tested the QM setup for a fully QM calculation on bulk water, which works 
fine. My main aim here is to reduce the energy drift for the QM/MM system, 
so is there any other aspect of the calculation should I take care to 
reduce the drift? Any suggested corrections for the QM/MM file would also 
be helpful.

Best Regards,
Mayank Dodia



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