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Hello Everyone,

I am new user or VASP. I want to calculate ground state energy of Gaseous molecules of H2, HCl and H2O. As i test case i have created xyz file of H2 molecule and using VESTA software i have created POSCAR file. I did the geometry optimisation and i got the energy(without entropy) of H2 -15.9325 eV. The POSCAR file has H2 molecule in 1x1x1 Angstrom. I have changed the cell size by 10x10x10 and after optimisation i got energy (without entropy) = -0.05844729 eV.
I want to know which one is true and is it what size of box i should choose to calculate the energy of any single gas molecule?

Regards,
Amar

No, your energies are not reasonable.
In this forum you can find many examples of calculations of molecules.
E.g. H2 molecule:
http://cms.mpi.univie.ac.at/vasp-forum/ ... ecule#p555
Reasonable box size for a small molecule is 8x8x8.
Take note that energy depends on: box size, k-points set, functional, ...