25
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 ...
- 7,281
15
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:
$...
- 763
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.
...
- 3,921
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 ...
- 256
12
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 ...
- 10.9k
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 ...
- 7,281
12
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 ...
- 6,567
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, ...
- 7,281
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, ...
- 2,332
11
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 ...
- 3,081
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 ...
- 665
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 ...
- 10.9k
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 ...
- 9,076
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 ...
- 7,957
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 ...
- 17.9k
10
votes
Accepted
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 ...
- 7,281
10
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 ...
- 17.9k
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 ...
- 10.9k
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-...
- 6,567
7
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 ...
- 3,238
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 ...
- 8,922
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 : ...
- 5,683
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. ...
- 3,084
7
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 ...
- 186
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 ...
- 940
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 ...
- 10.9k
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 ...
- 10.8k
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 ...
- 4,306
6
votes
Accepted
Do the cc/pc/def2 basis sets mathematically converge to the CBS limit, assuming exact CI/DFT?
"Assuming exact CI/DFT, can the sequences of the results of an ever increasing cardinal number of the cc/pc/def2 basis sets mathematically shown to converge exactly to the CBS limit, according to ...
- 33.5k
6
votes
What should be the criteria of convergence over ENCUT?
Preamble
From the VASP wiki, ENCUT specifies the cutoff energy for the plane-wave-basis set in eV. This is the same as ecutwfc ...
- 2,091
Only top scored, non community-wiki answers of a minimum length are eligible