22 votes

k-points and ENCUT convergence tests before or after relaxation?

This is a good question. In general, it often may not matter too much, but as with all aspects of numerical convergence, the only way for you to know for sure is to investigate. The concise answer is ...
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  • 6,852
14 votes

Python script for convergence test

Here is the simple bash script for testing kpoint energy convergence for VASP you can use same logic in python but you can also use this. ...
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  • 3,064
14 votes
Accepted

Python script for convergence test

There is an example in the pyiron documentation to calculate energy volume curves: https://pyiron.readthedocs.io/en/latest/source/notebooks/energy_volume_curve.html And the corresponding jupyter ...
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  • 256
13 votes

Doubt about energy convergence with different k points

Let's just remind ourselves what k point sampling is. It is used to approximate the integral of a function over the Brillouin zone by replacing the integral with a sum over a finite number of points: $...
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  • 522
12 votes

In DFT, if I complete a convergence test once, then want to study a different system with the same atoms, do I have to do a new convergence test?

As you might expect, the answer is: it depends. It really boils down to how different the two systems are. There are obvious cases where the settings will not transfer between two systems. The ...
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  • 6,852
11 votes

Which property should be used for a DFT convergence test? Is energy always the best, or should convergence be tested for a different property?

The question you should be asking yourself is: what properties do I want to compute? You, naturally, want to make sure that property is converged. For instance, if you wish to compute the band gap, ...
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  • 6,852
11 votes

How to make a k-point convergence test for a bulk structure with a lower k-point in the z-direction?

If you have already obtained satisfactory convergence with a (relatively) sparse k-point grid, there is no motivation to go for a denser grid. So if you have already achieved convergence with 12x12x4, ...
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  • 2,172
11 votes
Accepted

Increasing supercell size to increase K-points sampling

Short answer: Yes, doubling the volume of the simulation cell you will be able to effectively sample a finer $\mathbf{k}$-point grid to calculate the dielectric function. However, the calculation will ...
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  • 10k
10 votes

Python script for convergence test

This is the script I wrote for convergence testing in Quantum ESPRESSO using Bash and Python. According to Stefaan Cottenier, hydrostatic pressure on a unit cell is the property tested for convergence ...
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10 votes

Is total energy difference lower than 1 meV/atom good enough for k-point convergence?

When using a method like density functional theory we have to consider the accuracy of (i) the numerical solution of the problem and (ii) the physical model we use. The convergence you are refering to ...
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  • 10k
10 votes

k-points and ENCUT convergence tests before or after relaxation?

Andrew's answer covers a lot of other considerations but not some useful additional ones in the case of relaxing unit cell parameters in solids. If the unit cell is being optimized in addition to ...
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  • 2,881
10 votes

MD: How to estimate number of timesteps to converge a property

On the mathematical side, there exists no algorithm that estimates the time required to converge a property within any predefined accuracy, however low the accuracy is. The reason is simple. Suppose ...
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  • 6,418
10 votes

MD: How to estimate number of timesteps to converge a property

tl;dr: simulate for greater than N*period of oscillation To understand how I arrived at this answer consider that properties can be written as a function of the partition function Q. For example, the ...
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  • 7,754
10 votes
Accepted

Energy cut-off test meaning in atomic-like basis functions' DFT

There is no energy cut off test for calculations that employ atomic basis sets, in general: the calculation is well-defined with just the atomic basis set. For comparison, the Gaussian-basis PySCF ...
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  • 14.2k
10 votes
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Which properties of a material calculated using density functional theory are affected by smearing?

In brief, it will influence the electronic energy and thereby all properties derived from that. Too small a smearing width and you might have trouble converging the self-consistent field. Too large ...
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  • 6,852
10 votes
Accepted

In DFT, while studying a system using different exchange-correlation functional approximations, are we expected to check convergence for each method?

The short answer is "yes" you should check, however this is not as arduous as it might appear at first. Your initial convergence calculations will be fairly exhaustive, sweeping over a large range of ...
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  • 3,804
9 votes

Standard values for level of convergence

This is an excellent question! The reality is complicated even in LCAO calculations: every code has different defaults, which also depend on the run type. It seems that older LCAO codes simultaneously ...
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  • 14.2k
9 votes

In DFT, if I complete a convergence test once, then want to study a different system with the same atoms, do I have to do a new convergence test?

You should converge each calculation with respect to all the parameters; however, some parameters are relatively transferable between related systems. Let's consider the two main ones in a condensed-...
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  • 3,804
9 votes

How to make a k-point convergence test for a bulk structure with a lower k-point in the z-direction?

I agree with the answer provided by Xivi76. I just wanted to add that some codes have a very nice functionality that facilitates convergence: rather than explicitly writing out a $\mathbf{k}$-point ...
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  • 10k
7 votes

How to make a k-point convergence test for a bulk structure with a lower k-point in the z-direction?

As far as my knowledge this a relationship between k-points and lattice constants values. I'll give you an example of a layered hexagonal material WS2: Lattice constants : ...
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  • 5,546
7 votes

Which property should be used for a DFT convergence test? Is energy always the best, or should convergence be tested for a different property?

The hydrostatic pressure on a given unit cell is an experimentally observable property very sensitive to numerical precision. If this property is converged many other less sensitive properties will be ...
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  • 8,362
7 votes
Accepted

Why is Energy Cut-off and K-Point Convergence necessary in LCAO-based codes like SIESTA?

The k-point discretization has the same meaning in plane wave codes as LCAO based codes. In fact, it has the same meaning in all DFT related codes. It defines the integration of the Brillouin zone. ...
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  • 1,724
7 votes

Doubt about K point selection for 2x2x1 supercell

The KPOINTS have an inverse relationship with the real space lattice vectors. In either way, you won't go wrong. I would prefer to test it using 1x1x2, 1x1x3, 1x1x4, etc. This is simply because your c ...
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6 votes

Input constraints while comparing different DFT calculated properties for the same material but different atomic arrangement

The best strategy when performing convergence tests is to directly converge the quantity you are interested in. This "quantity" can be a straight-forward physical property, like the band gap ...
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  • 10k
6 votes
Accepted

k-points and ENCUT convergence tests before or after relaxation?

I do generally follow the follwoing: Keep basic tags EDIFF, EDIFF to 1E-07 and 1E-06 (or -0.005) generally. i do use defaults if i want to do a faster run just for checking. and i dont know how can ...
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  • 146
6 votes

In DFT, while studying a system using different exchange-correlation functional approximations, are we expected to check convergence for each method?

I don't know if I can give you a definitive response, but I would like to give you some clues based on my experience working with DFT, with Quantum ESPRESSO to be more specific. As a thumb rule, I ...
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6 votes

How to make a k-point convergence test for a bulk structure with a lower k-point in the z-direction?

I will try to give the most practical answer, the reality of "is this converged" is that you cannot know without checking by going past it. You say that you would like to save time doing ...
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6 votes

MD: How to estimate number of timesteps to converge a property

I wasn't going to answer, because there is no answer, but, anyways... First: common practice is... Common practice is to frequently measure the property you are after, as well as others, such as ...
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  • 3,810
5 votes

Which properties of a material calculated using density functional theory are affected by smearing?

You can make a convergence test to obtain reasonable results. Usually, for k-sampling and energy cutoff, you can take some values from experiences (of course, you can also make convergence tests). (a)...
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  • 13.6k
5 votes
Accepted

Does QC convergence mode in Gaussian make a difference?

The SCF=QC keyword in Gaussian actually pertains to the choice of algorithm on how find the solution in the SCF procedure while SCF=Tight is an option related to SCF cycle convergence. Gaussian SCF ...
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  • 116

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