QMMM_Surface: Ziegler-Natta catalysis¶

Sample directory: adf/QMMM_Surface/

This is an example of a Ziegler-Natta type catalytic system: a TiCl complex embedded in a MgCl surface with two organic substrates also attached to the surface. To make the computation faster, the QM/MM approach is applied. The QM part includes only the active site and a piece of the MgCl surface.

The computation is formally a geometry optimization, but to keep the sample doable in a reasonable time the sample performs only one geometry update step. In the optimization, all of the MgCl surface atoms are frozen.

The standard force field has been modified to accommodate this calculation. The modified force field file is part of the sample run script. In this modified file, bonds are defined between Mg-Cl atoms in the MM connection table. This results in some torsions where the atoms are collinear. To rectify this problem, the torsional potentials for these atoms are set to potential type ‘0’ (no potential).

There are no capping atoms mediating the bonds between the QM and MM regions because the boundary goes through the MgCl surface, which is ionically bound.

cat << eor > champ_de_force.ff
YBYL/TRIPOS FORCE FIELD FILE FOR ADF QM/MM
MODIFIED WITH UFF1.01 FOR Si Mg Ti Cl
L. Petitjean 15.11.1999
*************************************************************************

(Most of the contents of the modified force field file is omitted here. You quickly get the difference with the standard sybyl force field file in the ADF database by running a UNIX diff on the two files.

====================================
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$ADFBIN/adf  << eor
Title  ADF-QMMM in a surface study
NoPrint SFO, Frag, Functions

! keywords for calculation methods and optimization
XC
  GGA    BLYP
End

Geometry
  Optim          Cartesian Selected
  Iterations     1
  HessUpd        BFGS
  Converge       e=1e-4 grad=1e-3 rad=1e-2
  Step           rad=0.15
END

The ‘Iterations 1’ subkey specification in the Geometry block specifies that only one step in the optimization is carried out.

BeckeGrid
  Quality Basic
End

SCF
  Iterations  250
  Converge    1E-6 1E-6
  Mixing      0.2
  DIIS        N=10 OK=0.5 cyc=5 CX=5.0 BFAC=0
End


! keywords for molecule specification
Charge 0 0

Atoms Cartesian
    1 Mg    x1   y1   z1

(all other atoms in the Atoms block omitted here)

End

GeoVar
   x1=.00000 F
   y1=.00000 F
   z1=.00000 F
   x2=.00000 F
   y2=1.72129 F
   z2=1.82068 F
   x3=.00000 F
   y3=.00000 F
   z3=-3.64100 F
   x4=.00000 F
   y4=-1.72130 F
   z4=-1.82068 F
   x5=.00000 F
   y5=1.72130 F
   z5=-1.82032 F
   x6=.00000 F
   y6=1.72130 F
   z6=-5.46132 F
   x7=2.53903
   y7=.03004
   z7=-3.50645
   x8=2.50628
   y8=-.07048
   z8=-.10022
   x9=2.63009
   y9=3.50093
   z9=-3.02634
 ...

Many of the coordinates have a ‘F’ after their initial value specification under Geovar, indicating that these coordinates will be kept frozen during optimization.

The remaining initial value specifications are omitted here.

END
QMMM
   OPTIMIZE
     MAX_STEPS 3000
     MAX_GRADIENT 0.01
     METHOD BFGS
     PRINT_CYCLES 100
   SUBEND

   FORCE_FIELD_FILE champ_de_force.ff

The local file ‘champ_de_force.ff’ is used as force field file. Of course, this is the file we’ve just set up in the run script.

OUTPUT_LEVEL=1
WARNING_LEVEL=1
ELSTAT_COUPLING_MODEL=1

MM_CONNECTION_TABLE
    1   Mg  QM    2    4    5    8   58   60
...

Contents of the MM_Connection_Table block is omitted.

SUBEND
CHARGES
  1    .957
  2    -.608
  3    1.017
  4    -.411
  5    -.561
...

Initial charges are specified for (all) the atoms. Whether or not the charges on the QM (and LI) atoms are used depends on the type of electrostatic coupling between the QM and MM system. See the rest of the QM/MM manual for details.

   SUBEND
END

Fragments
  Ti t21.Ti
  Cl t21.Cl
  Mg t21.Mg
  C  t21.C
  H  t21.H
End


End Input
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