Nuclephile Substitution CH3Cl - SG

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Task

In this example the nucleophile substitution of a Cl^- by another Cl^- in CH₃Cl is simulated using a slow growth approach.

Input

POSCAR

   1.00000000000000
     12.0000000000000000    0.0000000000000000    0.0000000000000000
     0.0000000000000000    12.0000000000000000    0.0000000000000000
     0.0000000000000000    0.0000000000000000    12.0000000000000000
C H Cl
   1   3   2
direct
0.53294865 0.56575027 0.49613388
0.53110276 0.65294003 0.50241434
0.44611198 0.52863033 0.51450056
0.58463838 0.52611078 0.55968644
0.32726066 0.74478226 0.64936301
0.57915789 0.51894916 0.36275174

KPOINTS

Automatic
 0
Gamma
 1  1  1
 0. 0. 0.

• For isolated atoms and molecules interactions between periodic images are negligible (in sufficiently large cells) hence no Brillouin zone sampling is necessary.

PREC=Low
EDIFF=1e-6
LWAVE=.FALSE.
LCHARG=.FALSE.
NELECT=22
NELMIN=4
LREAL=.FALSE.
ALGO=VeryFast
ISMEAR=-1
SIGMA=0.0516

############################# MD setting #####################################
IBRION=0                                           # MD simulation
NSW=1000                                           # number of steps
POTIM=1                                            # integration step
TEBEG=600                                          # simulation temperature
MDALGO=11                                          # md with Andersen thermostat
ANDERSEN_PROB=0.10                                 # collision probability
LBLUEOUT=.TRUE.                                    # write the BM stuff on the output
INCREM=-1e-4                                       # rate at which CV increases each step
##############################################################################

ICONST

R 1 5 0
R 1 6 0
S 1 -1 0

Calculation

In a slow growth simulation, an approximation of the collective variable is increased by the value INCREM every time step. In order for the transformation between the initial and final state to be reversible, the value of INCREM must be infinitesimaly small. In practice we are limited by the desired length of trajectory causing that irreversible rather than reversible work is computed in a slow-growth simulation.

Running the calculation

The mass for hydrogen in this example is set 3.016 a.u. corresponding to the tritium isotope. This way larger timesteps can be chosen for the MD.

For practical reasons, we split our (presumably long) meta dynamics calculation into shorter runs of length of 1000 fs (NSW=1000; POTIM=1). This is done automatically in the script run which looks as follows:

#!/bin/bash 

runvasp="mpirun -np x executable_path"

# ensure that this sequence of MD runs is reproducible
cp POSCAR.init POSCAR
cp INCAR.init INCAR
rseed="RANDOM_SEED =         311137787                0                0"
echo $rseed >> INCAR

i=1
while [ $i -le 50 ] 
do

  # start vasp
  $runvasp

  # ensure that this sequence of MD runs is reproducible
  rseed=$(grep RANDOM_SEED REPORT |tail -1)
  cp INCAR.init INCAR
  echo $rseed >> INCAR

  # use the last configuration generated in the previous
  # run as initial configuration for the next run
  cp CONTCAR POSCAR

  # backup some important files
  cp REPORT REPORT.$i
  cp vasprun.xml vasprun.xml.$i

  let i=i+1
done

Download

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