Band gap using conventional and primitive cells

[IBRAHIM]

Dear VASP admin,

We tried to model a spinel cobalt structure with conventional and primitive cells. After full relaxation, we performed single point calculation using HSE06 to get the band gap.
However, we found the structure to be metallic with the primitive cell while it is semiconductor with the conventional cell.

In the case of the conventional cell, the electronic convergence was down to 1E-03 at step 25, then back to 1E+00 and start to reconverge again until the electronic steps limit "60". Then, we recontinue with wavecar and the calculation converges with a bandgap of about 0.8 eV.

In the case of the primitive cell, the calculation is converged in 28 steps with a metallic behavior.

-Why such different results from both cases, which one we should rely on?

-and what happened with the conventional cell's run, is it normal to converge up to some degree then return back and reconverge again?

Many thanks in advance,
Ibrahim

[martin.schlipf]

I need to look into this in more detail but from a quick glance here is what is happening: The conventional cell and the primitive cell converge towards the same minimum. Then around step 27 something disrupts the trajectory of the conventional cell and pushes it towards a different minimum. That other minimum has a lower energy. Currently I can only speculate what is the cause of this, perhaps there is some symmetry break that is only realized in the more complex cell. What you could do is run the conventional cell for only 25-27 iterations. If you inspect the state there, I suspect it will resemble the one in the primitive cell. You can also look at the orbital occupancies and see whether a pattern is established that you can transform to the primitive cell.

Currently, I would expect the conventional cell to give the correct results because the final energy is much smaller.

[IBRAHIM]

Thank you very much for your reply.

We will try the conventional cell up to 25 steps although it is very time consuming.

Please, we have two more questions:

-Should we expect that in our case, the energy of the conventional cell is about 4 times that of the primitive cell, if they are at the same minimum?

Also, for the conventional cell's run, in slurm output, there is:

CGA: 58 -0.464946395712E+03 -0.58732E-02 -0.58781E-02 3600 0.246E-01 0.380E-02
gam= 0.667 trial= 0.288 step= 0.275 mean= 0.288
gam= 0.000 g(H,U,f)= 0.109E-01 0.370E-02 0.172E-17 ort(H,U,f) =-0.246E-02 0.353E-02 0.678E-17
SDA: 59 -0.464950126123E+03 -0.37304E-02 -0.37371E-02 3600 0.146E-01 0.107E-02
gam= 0.000 trial= 0.288 step= 1.574 mean= 0.288
gam= 0.000 trial= 1.869 step= 1.869 mean= 0.454
gam= 0.000 g(H,U,f)= 0.369E-01 0.129E-01 0.229E-16 ort(H,U,f) =-0.148E-01-0.986E-03 0.558E-17
CGA: 60 -0.464950367313E+03 -0.24119E-03 -0.20402E-02 3600 0.498E-01-0.158E-01
gam= 0.000 trial= 0.524 step= 0.415 mean= 0.524
final diagonalization

-As you can see, is it normal such SDA step "59" and the following two lines for trial?

Many thanks in advance,
Ibrahim

[martin.schlipf]

-Should we expect that in our case, the energy of the conventional cell is about 4 times that of the primitive cell, if they are at the same minimum?

Yes, at least for setups with similar convergence parameters. In particular you should use a coarser k-point mesh for the conventional cell than for the primitive one.

Also, for the conventional cell's run, in slurm output, there is:

CGA: 58 -0.464946395712E+03 -0.58732E-02 -0.58781E-02 3600 0.246E-01 0.380E-02
gam= 0.667 trial= 0.288 step= 0.275 mean= 0.288
gam= 0.000 g(H,U,f)= 0.109E-01 0.370E-02 0.172E-17 ort(H,U,f) =-0.246E-02 0.353E-02 0.678E-17
SDA: 59 -0.464950126123E+03 -0.37304E-02 -0.37371E-02 3600 0.146E-01 0.107E-02
gam= 0.000 trial= 0.288 step= 1.574 mean= 0.288
gam= 0.000 trial= 1.869 step= 1.869 mean= 0.454
gam= 0.000 g(H,U,f)= 0.369E-01 0.129E-01 0.229E-16 ort(H,U,f) =-0.148E-01-0.986E-03 0.558E-17
CGA: 60 -0.464950367313E+03 -0.24119E-03 -0.20402E-02 3600 0.498E-01-0.158E-01
gam= 0.000 trial= 0.524 step= 0.415 mean= 0.524
final diagonalization

-As you can see, is it normal such SDA step "59" and the following two lines for trial?

The origin of this, is the following part in the VASP source code (rot.F):

Code: Select all

           WRITE(IO%IU0,10) GAMMA,BTRIAL,BOPT*BSTEP,AVERAG
   10 FORMAT(' gam=',F6.3,' trial=',F6.3,'  step=',F6.3,' mean=',F6.3)
   

! INTERATE_PREC requires at least 3 energy calculations
!    and seeks the minimum from these 3 energies
! apply this if BOPT gets unreasonably large (indicating 
! non-quadratic behaviour or some other trouble)
           IF (.NOT. ITERATE_PREC .AND. BOPT<5) THEN
              IFLAG=0
           ELSE
              ! maximum step size is 5 times the trial step
              ! there we will have data points at 4, 8 and 12 times the original step size
              BOPT=MIN(BOPT,5._q)
           ENDIF

The WRITE statement prints the line you see in the output. Then there is a check whether the desired step is less than 5 times larger than the trial step. If that is the case, IFLAG is set to 0 indicating that we are done searching for the appropriate step size. However in your case the ratio is 1.574/0.288 = 5.465 so VASP will do some more work to determine the optimal step size. Then it enters the routine again leading to the second line in the output.