[CP2K-user] Question about FIXED_ATOMS in MM simulation

[sm...@gmail.com]

Dear all,
I'm trying to create a cavity in Ar solution. (Ar system is meaningless. 
I'm just testing the function of input) I define atom V to represent the 
cavity center and set a repulsive nonbonded potential between V and Ar 
atoms. I use FIXED_ATOMS to fix V, however, I found V slightly moves during 
the MD process. I wonder how CP2K achieves FIXED_ATOMS in code? Is the 
force reset to be zero at every step?
The following is my input file and .xyz file is in the attachment.
&GLOBAL
        PROJECT Ar
        RUN_TYPE MD
        PRINT_LEVEL LOW
&END GLOBAL

&FORCE_EVAL
METHOD FIST
&SUBSYS
&CELL
ABC 16.0  16.0  16.0
ALPHA_BETA_GAMMA 90.0 90.0 90.0
&END CELL
&TOPOLOGY
COORD_FILE_FORMAT XYZ
COORD_FILE_NAME all.xyz
&END TOPOLOGY
&END SUBSYS
&MM
&FORCEFIELD 
                &SPLINE
                EMAX_SPLINE 1000000
            &END SPLINE
&CHARGE
ATOM Ar
CHARGE 0.0
&END CHARGE
&CHARGE
ATOM V
CHARGE 0.0
&END CHARGE
&NONBONDED
&GENPOT
ATOMS V Ar
FUNCTION A*((S/r)^12-2*(S/r)^6+0.25)
VARIABLES r
PARAMETERS A S
VALUES 0.00104506265 6.0
RCUT 3.0
&END GENPOT
                &LENNARD-JONES
                     atoms Ar Ar
                     EPSILON [K_e] 119.8
                     SIGMA   [angstrom]   3.401
                     RCUT    [angstrom]  8.0
                 &END LENNARD-JONES
                &LENNARD-JONES
                     atoms V V 
                     EPSILON [K_e] 0.0
                     SIGMA   [angstrom]   3.401
                     RCUT    [angstrom]  8.0
                 &END LENNARD-JONES
&END NONBONDED
&END FORCEFIELD
&POISSON
PERIODIC NONE
      &EWALD
        EWALD_TYPE none
      &END EWALD
    &END POISSON
&END MM
&END FORCE_EVAL

&MOTION
&CONSTRAINT
        CONSTRAINT_INIT TRUE
&FIXED_ATOMS
COMPONENTS_TO_FIX XYZ
LIST 1
&END FIXED_ATOMS
&END CONSTRAINT
  &MD   
    COMVEL_TOL 2.0E-6    
    ENSEMBLE NVT
    STEPS 10000
    TIMESTEP 0.5
    TEMPERATURE 330.0
    TEMP_TOL 50.0
    &THERMOSTAT
      &NOSE
        LENGTH 3
        YOSHIDA 3
        TIMECON 1000.0
        MTS 2
      &END NOSE
    &END THERMOSTAT
  &END MD
&PRINT
   &TRAJECTORY
     &EACH
      MD 10
     &END EACH
   &END TRAJECTORY
&END PRINT
&END MOTION
   
Sincerely,
Mengzi Zhou
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