Constrained MD using a microcanonical ensemble

Description: Compare adsoption of H2O on TiO2[110] using a simple model structure: the model surface consists of 2 layers TiO2, (1x1)

• The bottom layer of the slab will be kept frozen
• 2 setups will be tested:
  • Standard relaxation, minimizing the Hellmann-Feyman forces
  • Constrained MD, fixing the bond lengths and angle of H2O, the system is coupled to an ANDERSEN thermostat:
    • Microcanonical NVE ensemble (no collisions with the thermostat)
    • Canonical ensembe at T= 10K, to be close to the standard relaxation (0K)

To keep the computing time reasonable, the number of steps in the MD is limited to 100 (100 fs) which implies that the MD is NOT CONVERGED.

• INCAR for standard relaxation

SYSTEM = H2O_TiO2
ENMAX = 400
ISMEAR =  2
SIGMA = 0.05
EDIFF = 1e-6
EDIFFG = -0.05
IBRION = 2
POTIM = 0.5
NSW = 200

• INCAR for constrained MD using a microcanonical ensemble

SYSTEM = H2O_TiO2
ENMAX = 400
ISMEAR = 2
SIGMA = 0.05
ISMEAR = 0
EDIFF = 1e-6
EDIFFG = -0.05
IBRION = 0
POTIM = 1.
MDALGO = 1    # Andersen Thermostat
TEBEG = 10; TEEND = 10
NSW = 100    

• ICONST for constrained MD using a microcanonical ensemble

R 7 8 0
R 7 9 0
A 8 7 9 0

• INCAR for constrained MD using a canonical ensemble

SYSTEM = H2O_TiO2
ENMAX = 400
ISMEAR = 2
SIGMA = 0.05
ISMEAR = 0
EDIFF = 1e-6
EDIFFG = -0.05
IBRION = 0
POTIM = 1.
MDALGO = 1    # Andersen Thermostat
ANDERSEN_PROB = 0.9
TEBEG = 10; TEEND = 10
NSW = 100

• ICONST for constrained MD using a canonical ensemble

R 7 8 0
R 7 9 0
A 8 7 9 0

• POSCAR

TiO2+H2O                                    
   1.00000000000000     
     4.61949   0.00000    0.00000
     0.00000   4.61949    0.00000
     0.00000   0.00000   14.7788
   Ti   O  H
     2     5     2
Selective
Direct
  0.00000  0.00000  0.00000    F F F
  0.50000  0.50000  0.10000    T T T
  0.30374  0.30374  0.00000    F F F
  0.69625  0.69625  0.00000    F F F
  0.19625  0.80374  0.10000    T T T
  0.80374  0.19625  0.10000    T T T
  0.50000  0.50000  0.31500    T T T
  0.37720  0.62280  0.35881    T T T
  0.62280  0.37720  0.35881    T T T

• KPOINTS

Automatically generated mesh
       0
Gamma
 5 5 1

• run.sh

#
# To run VASP this script calls $vasp_std
# (or posibly $vasp_gam and/or $vasp_ncl).
# These variables can be defined by sourcing vaspcmd
. vaspcmd 2> /dev/null
    
#
# When vaspcmd is not available and $vasp_std,
# $vasp_gam, and/or $vasp_ncl are not set as environment
# variables, you can specify them here
[ -z "`echo $vasp_std`" ] && vasp_std="mpirun -np 8 /path-to-your-vasp/vasp_std"
[ -z "`echo $vasp_gam`" ] && vasp_gam="mpirun -np 8 /path-to-your-vasp/vasp_gam"
[ -z "`echo $vasp_ncl`" ] && vasp_ncl="mpirun -np 8 /path-to-your-vasp/vasp_ncl"
    
#
# The real work starts here
#
      
rm results.dat 
      
drct=$(pwd)
      
for i in std_relaxation constrMD_microcanonical constr_MD_canonical
do
  cd $drct/$i
  ln -s ../POTCAR .
  ln -s ../POSCAR .
  ln -s ../KPOINTS .
  $vasp_std
/bin/rm CHG* WAVECAR
done

Tor run the calculations use (and modify if necessary) the run.sh script

Download

h2o_on_tio2.tgz, sub-folder constrMD_microcanonical

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