Different between H2 and D2 molecule energy

[rubi_agrawal]

I have properly adjusted the mass values (POMASS) for deuterium (D) and hydrogen (H) in the molecule files, POMASS = 2 and 1 in POTCAR file ,yet the energy remains identical for H2 and D2 molecule optimizations. Is there a specific step or parameter I might be overlooking? and my INCAR file is:

Code: Select all

LSCALAPACK= .FALSE.
#NCORE= 4
NPAR = 4

LPLANE= .TRUE.
NSIM= 1
#Electronic relaxation
ENCUT=400     #
IALGO= 48
EDIFF= 0.1E-05
ISMEAR= 1
SIGMA= 0.1
# Ionic relaxation
EDIFFG= -0.01
NSW= 300
IBRION= 1
POTIM= 0.3
#genral
ISYM= 2
PREC= Normal
NELM= 300
NGX= 27
NGY= 27
NGZ= 133
LREAL= .FALSE.
ISIF= 2
ISPIN= 2
NUPDOWN= 0
ISMEAR= 1
SIGMA= 0.1
LCHARG= .FALSE.
LWAVE= .FALSE.
#for PBE function
GGA = PE

[manuel_engel1]

Adjusting the masses of the ions changes the kinetic energy, but that is not included in the free energy. So unless you are doing, for example, a molecular dynamics run, I wouldn't expect to find any difference between the two calculations.

As a sidenote, it might be more convenient to change the masses of the ions via the POMASS tag in the INCAR file without having to modify the POTCAR file.

[rubi_agrawal]

Okey, I understood but the dissociation energy is different (2 KJ/mol). so how can I calculate then?

[manuel_engel1]

I'm afraid I don't quite understand your question. Are you trying to calculate the dissociation energies of the two molecules?

[rubi_agrawal]

Okey, this is complete question i want to calculate the adsorption energy of H2 and D2 on Pd(111) surface this is already reported in this paper ( pubs.acs.org/doi/10.1021/acs.jpcc.2c04567 ) if POMASS not depend (mean POSCAR and POTCAR file is same) then how to come the adsorption energy is different.

[alex]

Hi there, their abstract (https://pubs.acs.org/doi/10.1021/acs.jpcc.2c04567) says:
'Thermal recombinative desorption rates of HD on Pd(111) and Pd(332) are reported from transient kinetic experiments performed between 523 and 1023 K. A detailed kinetic model accurately describes the competition between recombination of surface-adsorbed hydrogen and deuterium atoms and their diffusion into the bulk. By fitting the model to observed rates, we derive the dissociative adsorption energies (E0, adsH2 = 0.98 eV; E0, adsD2 = 1.00 eV; E0, adsHD = 0.99 eV) as well as the classical dissociative binding energy ϵads = 1.02 ± 0.03 eV, which provides a benchmark for electronic structure theory. In a similar way, we obtain the classical energy required to move an H or D atom from the surface to the bulk (ϵsb = 0.46 ± 0.01 eV) and the isotope specific energies, E0, sbH = 0.41 eV and E0, sbD = 0.43 eV. Detailed insights into the process of transient bulk diffusion are obtained from kinetic Monte Carlo simulations.'

So a) you are talking about a measurement with error bars, which are given b) already in the abstract with +/- 0.01 eV.
So speaking chemistry here, we are talking about the same numbers for H2, D2 and HD adsorption energy.

Best regards,

alex