# Calculation of Gibbs free energy of Mg+ ion using VASP [closed]

I am trying to replicate the $$\Delta G_0$$ part from this paper.

According to the paper, $$\Delta G_0$$ is 30% of the difference between atomic energy for per $$\ce{Mg}$$ atom in the bulk structure and the energy of one $$\ce{Mg+}$$ ion. I was able to calculate the bulk energy per atom as -1.505 eV and verified it with respect to cohesive energy obtained in multiple papers (energy of isolated atom is -5.527e-4, so can be neglected). However, I am not getting consistent results with respect to the $$\ce{Mg+}$$ ion. According to the paper, I am supposed to get $$\Delta G_0$$ as 3.03 eV. This implies that the difference should be around 10.1 eV (or, energy of Mg+ ion should be 10.1-1.505 = 8.595 eV). However, I am getting it as 7.8877 eV.

I used the following tags in INCAR:

SYSTEM = Mg
ISTART=0         # start new job
ICHARG=2         # 2 for atom
INIWAV=1         # initial wave function
ENCUT= 250.00 eV # Energy cut-off for calculation
PREC = Accurate    # Normal Precision
LREAL= AUTO   # real space projection .FALSE.
LCHARGE= .FALSE  # no charge file output
LWAVE= .FALSE    # no wavefile output
ALGO=FAST
NCORE=8
KPAR=1
ISMEAR = 1;     # tetrahedron method with Bloech 1 correction
SIGMA = 0.2      # smearing value
ISIF = 2;        # 2- scf CALCULATION/IONIC MOVEMENT # 3 for volume relax
IBRION=2         # cg
NSW=200           # number of step for ionic update
EDIFF=1E-5       # stopping criteria for electronic update
EDIFFG=-0.01
SYMPREC=1e-6
NELECT=1
LMONO = .TRUE. # monopole corrections


Also, I have used Gamma centred Monkhorst pack grid of 1x1x1.

Please suggest corrections or any inputs to improve the result.

• +1. Good first question and welcome to our new community! Thank you for contributing your question here, and we hope to see much more of you in the future!! I have made some edits, can you please go through your question and make the same edits throughout the rest of the question? Also, by M and M+, do you mean Mg and Mg+? – Nike Dattani Mar 30 at 16:32
• Hi Nike. Thanks for the edits. I have changed M and M+ to Mg and Mg+ as well. Waiting for suggestions on code :) – Pragyandipta Mishra Mar 31 at 6:04
• Happened to see that this was deleted. Even though this hasn't gotten an answer for awhile, its not necessary to delete it. Also, looking at the paper, the energy cutoff they use is 520 eV. Is the 250 eV just a typo in your post or an actual typo in your input file? – Tyberius Apr 19 at 15:11
• @Tyberius I think we should let the user know though, that if they still do want it deleted, they are welcome to. We don't want to force people (especially those using their real name) to have material associated with their username visible to the public, if they don't want it that way. – Nike Dattani Jun 11 at 21:25