The adsorption energy is bonus?

Message

lanjh · #1 Post by **lanjh** » Wed Jun 27, 2007 12:54 am

hellow!
I just calculated the adsorption energy of NH3 on Pt(111) surface/ But I found the adsorption energy is bonus~ This is of course a wrong results. Because others had done the same jobs, and obtained a value about -0.7eV.
The calculation was done as follows:
First, optimize the supercell of NH3 and Pt system.
Then, delete Nh3 from the above supercell. and optimized
At last, The NH3 was optimized with the same supercell
The three jobs was down with totally the same parameter.

Can anyone tell me where is the problem?

lanjh · #2 Post by **lanjh** » Wed Jun 27, 2007 6:06 am

Sorry ,I have made a mistake. The calculated adsorption energy is not bonus. So I didn't not why. SOS

lanjh · #3 Post by **lanjh** » Thu Jun 28, 2007 1:15 pm

Can anyone give me some advice?

lanjh · #4 Post by **lanjh** » Thu Jun 28, 2007 2:26 pm

SYSTEM=Pt(111)_top
ENCUT=420
ENAUG=700
ISTART=0;ICHARG=2
ISMEAR=2; SIGMA=0.2
NSW=100
POTIM=0.3
IBRION=2
ALGO=V
GGA=91
EDIFF=1E-5
EDIFFG=-1E-2
PREC=Medium
LREAL=Auto
LWAVE=.FALSE.

Maybe I need provide more information. This is my INCAR.

#5 Post by **admin** » Mon Jul 02, 2007 8:01 am

please define what you mean by bonus . ..
--) do you take the balance correctly (final state - initial stae)
--) maybe the final state is a metastable state, not the equilibrium state of NH3 on Pt. Please check whether the geometry of that result is reasonable.
--) if NH3 is adsorbed on one surface only, you have to add dipole corrections
--) check whether ALGO=Fast gives the same results.

lanjh · #6 Post by **lanjh** » Tue Jul 17, 2007 12:31 am

Sir, my problem still exists. Now I list my results .
For Pt/NH3 system, energy without entropy= -101.262864
For Pt slab, energy without entropy= -83.484689
For NH3 with the same box: energy without entropy= -19.084103
The parameters is as below: SYSTEM=Pt(111)_topa
ENCUT=420
ENAUG=700
ISTART=0;ICHARG=2
ISMEAR=2; SIGMA=0.2
NSW=100
POTIM=0.5
IBRION=2
ALGO=V
GGA=91
EDIFF=1E-5
EDIFFG=-1E-2
PREC=Accurate
LWAVE=.FALSE.
LCHARG=.FALSE.
#IDIPOL=3

When using the item ALGO=FAST,
energy without entropy= -101.261532
when the dipole corrections is considered,
energy without entropy= -100.878408

Now I am confused. I can check the geometry. The N-Surface distance in previous report is 2.16 A. In my calculation is 2.127. I thank the calculated structure is normal.
What is wrong with my work?

#7 Post by **admin** » Sat Jul 21, 2007 4:54 pm

are you sure that the adsorption site you considered is the most stable one?

radny · #8 Post by **radny** » Wed Oct 17, 2007 6:21 am

Try this:
First, optimize the supercell of NH3/Pt adsorbed system.
Then, SEPARATE Nh3 from the substrate within the same supercell (the substrate should be a free, relaxed surface and the molecule like in a gas phase;)
Calculate the adsorption energy as E(adsorbed-system) - E(separated-system).

From our experience with VASP your method and the method just described give different results. I would like to know why. Any ideas?