Different in ground state energy (E0) from spin and non-spin polarized

Message

PRKN · #1 Post by **PRKN** » Fri Jul 06, 2012 7:32 am

Hi Everyone,

I am a new VASP user. Currently I try to follow the VASP hands-on tutorial.

In the hands-on, the first two examples are oxygen atom without and with spin polarized respectively. I noticed that on the last line of the OZICAR file, the value E0 from non-spin and spin polarized systems are different. (E0 = -.16001392E+00 for non-spin and E0 = -.15948179E+01).

My question is which one is the most correct one?

Note: I try to investigate more on this issue by calculated the E0 for hydrogen atom. I've found that the calculation with spin polarized gives more correct answer; the ground state energy is very close to -13.6 eV from the analytical value of ground state of hydrogen atom. While the non-spin polarized calculation gives the value around -12.5 eV.

Thank you very much.

Danny · #2 Post by **Danny** » Fri Jul 06, 2012 2:44 pm

the spin polarized value should be more correct since it takes spin into account. Then again for the O atom the difference is 5meV...so this difference could be within the error of your accuracy. (increase ENCUT and decrease EDIFF to see what happens)

PRKN · #3 Post by **PRKN** » Sat Jul 07, 2012 5:57 am

Thank you very much. I will try to include the ENCUT and EDIFF as you suggested.

However, I think the different in energy in case of oxygen atom is more than 5meV. Because
(-.15948179E+01) - (-.16001392E+00)
= (-1.5948179E+00) - (-.16001392E+00)
= -1.434804 eV

PRKN · #4 Post by **PRKN** » Tue Jul 10, 2012 6:03 am

Sorry for late reply. I follow your suggestion by increasing ENCUT and EDIFF. The result is the value E0 in OSZICAR file improves, but still not close to the case that we include ISPIN into the INCAR file.

I calculate oxygen atom system by using input file provided by VASP Hands on Session I, folder 1_1_Oatom. I only modified INCAR file. Below is my modified INCAR file

INCAR (modified)
-----------------------------------------------------------
! Parameters provided by VASP Hands on Session I
SYSTEM = O atom in a box
ISMEAR = 0 ! Gaussian smearing

!Additional parameters
ENCUT = 400
EDIFF = 10E-07
-----------------------------------------------------------

Below is the comparison on the result in the last line of OSZICAR file in case with and without additional parameters in INCAR file (additional parameters are ENCUT and EDIFF)

OSZICAR (without additional parameters)
-----------------------------------------------------------
1 F= -.31463718E+00 E0= -.16037486E+00 d E =-.308525E+00
-----------------------------------------------------------
OSZICAR (with additional parameters)
-----------------------------------------------------------
1 F= -.31467650E+00 E0= -.16041418E+00 d E =-.308525E+00
-----------------------------------------------------------