# Planewave cutoff energy convergence

• mayerz
In summary, the speaker is having trouble achieving convergence with the planewave cutoff energy for a system with 2 atoms. They have successfully determined the minimum k-point mesh, but when computing the total energy with varying PWcE, it only seems to converge at higher values. They are using ultrasoft pseudopotentials and have not allowed the atoms to relax, so the only convergence criteria is the total energy. They are considering changing to a relaxation calculation. The speaker has also conducted calculations with higher PWcE values and the plot seems to be almost asymptotic at a cutoff of 880 eV. They are unsure if they can take the 400 eV value as the planewave cutoff energy and are using a 6

#### mayerz

Hello everyone,

I have some problems with the convergence for the planewave cutoff energy, I tried to find the correct PWcE for a system with 2 atoms, I've determined the minimum k-point mesh with success, but when I compute the Total energy varying the PWcE, it seems that the convergence will be achieved till too high values of PWcE, should I change the pseudopotentials for the atoms?

I use DACAPO,

I'm Mexican by the way, so sorry if I made a mistake while I wrote this.

best,

If the pseudopotential is hard then you may not achieve convergence until reaching to very high cutoff (~700+ eV). On the other extreme ultrasoft pseudopotentials tend to converge with much lower values. Another factor is the type of element you are studying.
Check the manual of your code and the recommended values of the pseudopotential.

Useful nucleus said:
If the pseudopotential is hard then you may not achieve convergence until reaching to very high cutoff (~700+ eV). On the other extreme ultrasoft pseudopotentials tend to converge with much lower values. Another factor is the type of element you are studying.
Check the manual of your code and the recommended values of the pseudopotential.

Thanks for your response, but in fact I'm using ultrasoft pseudopotentials (Vanderbilt), It seems that the Total energy converge at 400 eV but when I compute the total Energy with higher PWcE the tot E fall again to lower values. I didnt allow the atoms to relax, so the unique converge criteria is the total Energy, should I change to a relaxation calculation?

best

There is no need to conduct the energy convergence test in a relaxation simulation. A single point calculation is good enough to test energy convergence.

Can you tell us what the total energy difference per atom between the 400eV cutoff calculation and the more conservative one?

Useful nucleus said:
There is no need to conduct the energy convergence test in a relaxation simulation. A single point calculation is good enough to test energy convergence.

Can you tell us what the total energy difference per atom between the 400eV cutoff calculation and the more conservative one?

Of course! If I compute the total energy difference between the calculations done with a planewave cutoff energy of 320 and 400 eV , I get like 36 KJ/mol; something similar but in the interval of 400 - 480 eV (PWcE) I get about 4 KJ/mol; and the same with a PWcE range 480-700 eV, I get 17 KJ/mol (I've already done some calculations with higher pwce, up to 880 eV, and at this point (starting in 700) the plot seems to be almost asymptotic). But it seems to be a kind of weird to me, because I was hoping for a lower pwce value, it makes any sense?
could I take the 400 eV value as the planewave cutoff energy?

by the way I use a 6 atoms periodic model!

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