My Community

Hello, everybody!
I am a new user of VASP, and I also read the Hands-on tutorial courses.
One of the topics is adsorption, I have tried several times, but it doesn't work.
Here is my INCAR
ISTART=0
ICHARG=2
ISIF=2
IBRION=2
ISMEAR=0
SIGMA=0.1
LREAL=Auto
ALGO=Fast
LPLANE=.TRUE.
NSIM=4
NSW=30
ENCUT=400
My sunstrate is fixed, and the adsorbate is free, I want to observation the adsorption process, but in vain.
The system always keeps the adsorbate almost at the same position, and the adsorbate do relaxation itself, so I can't see any adsorption.
All I have see is two fragments at the origin position, and the adsorbate relax itself.
What is my problem?
Could anybody tell me?
Thanks in advance!

From your posting, I guess that your question concerns the adsorbate/substrate relaxation to the equilibrium geometry.
Your input (as far as INCAR is concerned) seems reasonable. So most probably, the starting geometry was
--) either aleady a very good guess
--) or (unfortunately) a local (but not the global) minimum on the potential hypersurface.
--) the shape of the potential hypersurface is very shallow
to check this please
--) if available, compare the adsorbate/substrate configuration with experimental data
(it may help if you provide more information than just INCAR about your calculation, INCAR is very unspecific)
--) if you are sure that your starting geometry is far from the equilibrium, increase POTIM (default=0.5) and do a few steps (NSW=3-5) to check whether a large trial step helps to surmount the energy barrier (in case your system shows activated adsorption)
it may help if you give more detailled information about your calculation, the INCAR file itself is very unspecific.

Thanks for your replying!
Here is my POSCAR

CNT 3x3_h2
1.0
17.0 0.0 0.0
0.0 17.0 0.0
0.0 0.0 10.0
24 2
Selective dynamics
Cartesian
7.899075031281 5.791452884674 3.150000000000 F F F
6.847000122070 7.612732887268 4.381065988541 F F F
4.743247032166 7.612185955048 3.150000000000 F F F
3.691509008408 5.790851116180 4.381065988541 F F F
4.743593215942 3.969571113586 3.150000000000 F F F
6.847345829010 3.970118045807 4.381065988541 T T T
7.406269073486 7.143881797791 3.150000000000 F F F
5.429059028625 7.861935138702 4.381065988541 F F F
3.818017959595 6.509446144104 3.150000000000 F F F
4.184323787689 4.438422203064 4.381065988541 F F F
6.161534786224 3.720369100571 3.150000000000 F F F
7.772574901581 5.072857856750 4.381065988541 T T T
7.899075031281 5.791452884674 5.612132930756 T T T
6.847000122070 7.612732887268 6.843188905716 F F F
4.743247032166 7.612185955048 5.612132930756 F F F
3.691509008408 5.790851116180 6.843188905716 F F F
4.743593215942 3.969571113586 5.612132930756 F F F
6.847345829010 3.970118045807 6.843188905716 T T T
7.406269073486 7.143881797791 5.612132930756 F F F
5.429059028625 7.861935138702 6.843188905716 F F F
3.818017959595 6.509446144104 5.612132930756 F F F
4.184323787689 4.438422203064 6.843188905716 F F F
6.161534786224 3.720369100571 5.612132930756 T T T
7.772574901581 5.072857856750 6.843188905716 T T T
12.341529 1.121128 5.612133 T T T
11.871356 0.550613 5.612133 T T T

First, I do the relaxation of the CNT, and then let the coordianets fix, except of the 6 carbon atoms which is nearby the H2 molecule.
KPOINTS is 1x1xn, n between 20~30.
POTCAR is the combination of C and H.
My thought is that the system which consists of two fragments, i.e. the supercell.
I do the calculation in order to find the stable state of the system, so I predict the H2 should move to the equilibrium position, i.e. the min. of the energy state.
Above is my original idea, but it has no use.
Or I must use the elastic band method to achieve my calculation?
Thank you very much!

the reason for this behaviour seems to be the very large distance between H2 and the CNT (more than 6.2 Ã… compared to a C--H distance of slightly more than 1Ã… eg. in organic molecules). Maybe H2 was trapped in a shallow physisorbed state. Please try a different starting geometry with a smaller CNT-H2 distance.