Dear all,<div>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?</div><div>The following is my input file and .xyz file is in the attachment.</div><div>&GLOBAL</div><div>        PROJECT Ar</div><div>        RUN_TYPE MD</div><div>        PRINT_LEVEL LOW</div><div>&END GLOBAL</div><div><br></div><div>&FORCE_EVAL</div><div><span style="white-space:pre"> </span>METHOD FIST</div><div><span style="white-space:pre">   </span>&SUBSYS</div><div><span style="white-space:pre">           </span>&CELL</div><div><span style="white-space:pre">                     </span>ABC 16.0  16.0  16.0</div><div><span style="white-space:pre">                        </span>ALPHA_BETA_GAMMA 90.0 90.0 90.0</div><div><span style="white-space:pre">               </span>&END CELL</div><div><span style="white-space:pre">         </span>&TOPOLOGY</div><div><span style="white-space:pre">                 </span>COORD_FILE_FORMAT XYZ</div><div><span style="white-space:pre">                 </span>COORD_FILE_NAME all.xyz</div><div><span style="white-space:pre">               </span>&END TOPOLOGY</div><div><span style="white-space:pre">     </span>&END SUBSYS</div><div><span style="white-space:pre">       </span>&MM</div><div><span style="white-space:pre">               </span>&FORCEFIELD </div><div>                &SPLINE</div><div>                EMAX_SPLINE 1000000</div><div>            &END SPLINE</div><div><span style="white-space:pre">                     </span>&CHARGE</div><div><span style="white-space:pre">                           </span>ATOM Ar</div><div><span style="white-space:pre">                               </span>CHARGE 0.0</div><div><span style="white-space:pre">                    </span>&END CHARGE</div><div><span style="white-space:pre">                       </span>&CHARGE</div><div><span style="white-space:pre">                           </span>ATOM V</div><div><span style="white-space:pre">                                </span>CHARGE 0.0</div><div><span style="white-space:pre">                    </span>&END CHARGE</div><div><span style="white-space:pre">                       </span>&NONBONDED</div><div><span style="white-space:pre">                                </span>&GENPOT</div><div><span style="white-space:pre">                                   </span>ATOMS V Ar</div><div><span style="white-space:pre">                                    </span>FUNCTION A*((S/r)^12-2*(S/r)^6+0.25)</div><div><span style="white-space:pre">                                  </span>VARIABLES r</div><div><span style="white-space:pre">                                   </span>PARAMETERS A S</div><div><span style="white-space:pre">                                        </span>VALUES 0.00104506265 6.0</div><div><span style="white-space:pre">                                      </span>RCUT 3.0</div><div><span style="white-space:pre">                              </span>&END GENPOT</div><div>                &LENNARD-JONES</div><div>                     atoms Ar Ar</div><div>                     EPSILON [K_e] 119.8</div><div>                     SIGMA   [angstrom]   3.401</div><div>                     RCUT    [angstrom]  8.0</div><div>                 &END LENNARD-JONES</div><div>                &LENNARD-JONES</div><div>                     atoms V V </div><div>                     EPSILON [K_e] 0.0</div><div>                     SIGMA   [angstrom]   3.401</div><div>                     RCUT    [angstrom]  8.0</div><div>                 &END LENNARD-JONES</div><div><span style="white-space:pre">                 </span>&END NONBONDED</div><div><span style="white-space:pre">            </span>&END FORCEFIELD</div><div><span style="white-space:pre">           </span>&POISSON</div><div><span style="white-space:pre">                  </span>PERIODIC NONE</div><div>      &EWALD</div><div>        EWALD_TYPE none</div><div>      &END EWALD</div><div>    &END POISSON</div><div><span style="white-space:pre">  </span>&END MM</div><div>&END FORCE_EVAL</div><div><br></div><div>&MOTION</div><div><span style="white-space:pre">      </span>&CONSTRAINT</div><div>        CONSTRAINT_INIT TRUE</div><div><span style="white-space:pre">                </span>&FIXED_ATOMS</div><div><span style="white-space:pre">                      </span>COMPONENTS_TO_FIX XYZ</div><div><span style="white-space:pre">                 </span>LIST 1</div><div><span style="white-space:pre">                </span>&END FIXED_ATOMS</div><div><span style="white-space:pre">  </span>&END CONSTRAINT</div><div>  &MD   </div><div>    COMVEL_TOL 2.0E-6    </div><div>    ENSEMBLE NVT</div><div>    STEPS 10000</div><div>    TIMESTEP 0.5</div><div>    TEMPERATURE 330.0</div><div>    TEMP_TOL 50.0</div><div>    &THERMOSTAT</div><div>      &NOSE</div><div>        LENGTH 3</div><div>        YOSHIDA 3</div><div>        TIMECON 1000.0</div><div>        MTS 2</div><div>      &END NOSE</div><div>    &END THERMOSTAT</div><div>  &END MD</div><div>&PRINT</div><div>   &TRAJECTORY</div><div>     &EACH</div><div>      MD 10</div><div>     &END EACH</div><div>   &END TRAJECTORY</div><div>&END PRINT</div><div>&END MOTION</div><div>   </div><div>Sincerely,</div><div>Mengzi Zhou</div>