[QMMM] H trapped in QM atoms when the QM region has negative charge (ECOUPL GAUSS)

[Shawn Yang]

Dear CP2K user,

Recently I am using QM/MM to do some MD calculations, the box include a 
negative QM box and MM water solvents, therefore the net charge is -1. For 
example, only CO2- in the QM region surrounded by TIP3P water. As far as I 
understand from the Laino's work in 2005, this unphysical problem has been 
solved by using a finite width density of charge on MM atoms.

However, the MM proton still trapped into the quantum oxygen atoms, and MM 
oxygen, also trapped into quantum carbon atom. The input for my QMMM part 
is attaed below, any suggestions would be appreciated. 

PS. When I test in pure water bulk (QM water + MM water) system, the result 
doesn't have this problem. So I guess if I could increase the width of MM 
atoms? Or apply an artifical replusive potential to avoid the trap? 

Many thanks for your help!

Xiaohui




  &QMMM
    &CELL
      PERIODIC NONE
      ABC [angstrom] 10 10 10
    &END CELL
    &WALLS
      TYPE NONE
    &END WALLS
    &PERIODIC
      GMAX 0.5
      &POISSON
        PERIODIC NONE
        POISSON_SOLVER MULTIPOLE
      &END
      &MULTIPOLE ON
        EWALD_PRECISION 1.0E-6
        RCUT 26
      &END
    &END PERIODIC
    &QM_KIND C
      MM_INDEX 1
    &END QM_KIND
    &QM_KIND Oq
      MM_INDEX 2 3
    &END QM_KIND
    &MM_KIND H
      RADIUS 0.44
    &END MM_KIND
    &MM_KIND O
      RADIUS 0.78
    &END MM_KIND
    E_COUPL GAUSS
    USE_GEEP_LIB 10
    NOCOMPATIBILITY
  &END QMMM
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