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Questions tagged [convergence-test]

Questions relating to convergence testing methods in matter modeling.

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Cut off convergence with Hubbard and long-range interactions on VASP

I have a multilayer structure formed by C,O,H,N and Cu, for a total of 174 atoms. I am thus using U+J Hubbard methodology as well as Grimme corrections within VASP code. I am doing some convergence ...
Laura's user avatar
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1 vote
0 answers
68 views

Can I use the convergence test results of ecutwfc, ecutrho, and k points obtained from a 2x2 graphene supercell for a larger graphene supercell?

Recently, I conducted convergence tests (ecutwfc, ecutrho, and k points) for a 2x2 graphene supercell, yielding results of 120 Ry, 300 Ry, and 10x10x1 respectively. (Here, we considered convergence ...
Thejan Hasaranga's user avatar
7 votes
1 answer
190 views

Convergence testing in DFT calculation

I have done convergence testing for MoS2 (2x2x1) monolayer structure in the following order: ecutwfc ecutrho k_points After setting these values, I would like to include vdW correction to the system ...
Joyal sunny's user avatar
3 votes
0 answers
39 views

How to choose k grid using pymatgen for VASP?

Before I start; sorry for this lengthy question. I am confused about choosing the correct set of K points for the K-convergence test for VASP calculations. The system of my interest has an ...
Nilabja Kanti Sarkar's user avatar
5 votes
2 answers
208 views

In plane-wave DFT, what needs to be converged and in what order?

Even though I use some Quantum ESPRESSO specific terminology here, this question applies to any plane-wave based pseudopotential DFT code. We know that before running an actual DFT calculation, a few ...
Abdul Muhaymin's user avatar
9 votes
1 answer
438 views

What should be the criteria of convergence over ENCUT?

For example, should a computation be considered to converge when the differences in energy per atom are smaller than a certain value?
蕭力諶's user avatar
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2 votes
1 answer
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Is there an analogue of the K-S theorem for double hybrids

In my previous question, I was basically asking whether the results of the double hybrid using the exact XC functional are the same as those of just the exact XC functional. Even that sentence is hard ...
Kanghun Kim's user avatar
7 votes
1 answer
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Is an "exact" double hybrid density the same as the "exact" DFT density?

Double hybrid approximate functionals have "unoccupied" Kohn-Sham orbitals in their formulations due to their MP2 component; however, the "exact functional" depends only on the &...
Kanghun Kim's user avatar
5 votes
2 answers
297 views

Do the cc/pc/def2 basis sets mathematically converge to the CBS limit, assuming exact CI/DFT?

The cc-, pc- and def2- basis sets are often described as "systematic", in the sense that the results of these basis sets at different cardinal numbers (i.e. the number of ζ's) can be ...
Kanghun Kim's user avatar
7 votes
2 answers
2k views

Does QC convergence mode in Gaussian make a difference?

I have been using Gaussian16 por metal complexes SCF single-point calculations involving triplets to investigate phosphorescence. I have actually run into trouble (very long computational time, even ...
Paul Logan's user avatar
9 votes
2 answers
567 views

Doubt about K point selection for 2x2x1 supercell

I have one doubt about K points selection for a supercell. I construct one 2X2X1 supercell with 64 atoms having a=11.19, b=11.19, c=5.59. For checking K points convergence test with energy should I ...
poonam sharma's user avatar
7 votes
1 answer
967 views

How accurate is ALGO= All in VASP?

I was facing convergence problems in my 2D system, but after I have used the ALGO = All tag the convergence problems have gone. I would like to know how accurate is ...
Chi Kou's user avatar
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6 votes
1 answer
511 views

Total Energy vs Energy Cut Off convergence

Why is my plot of 'Total Energy vs Energy cut off' is jagged. Previously ecutwfc was kept at 40, 60 and now 80. But still I am unable to figure why isn't it converging. Where as 'Total Energy vs K ...
147875's user avatar
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7 votes
1 answer
542 views

Monkhorst – Pack sampling: How many k points one should take

Reference: Density Functional Theory: A Practical Introduction, David Sholl, Janice A. Steckel, Chapter 3, Page No. 59 A useful rule of thumb is that calculations that have similar densities of k ...
147875's user avatar
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14 votes
1 answer
1k views

Doubt about energy convergence with different k points

During the convergence test of k points, I observed my energy start increasing instead of decreasing. I am doubtful is this correct or I am doing something wrong. can I take the appropriate K point in ...
poonam sharma's user avatar
14 votes
3 answers
602 views

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

If I want to equilibrate a NPT system until a property reaches its equilibrium value (let’s say the volume V), what is the common practice to have a rough estimate of the total simulation time ...
Barbaud Julien's user avatar
10 votes
1 answer
1k views

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

I have been using SIESTA code to perform some practicals in my college and I have run energy cut-off tests before calculating properties of semi-conductors or insulator. As far as I am concerned, ...
Paul Logan's user avatar
11 votes
1 answer
2k views

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

I am new to DFT-simulation and I wish to understand how it works behind the scene. As the title suggests I don't understand why LCAO-based codes have to define K-points and cut-off energies. I ...
Brentdb's user avatar
  • 113
13 votes
3 answers
3k views

Python script for convergence test

I want to do a convergence test of Ni using a Python script in Pyiron. Can someone please suggest how to write a script to test energy and k-mesh convergence?
Shalini's user avatar
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6 votes
1 answer
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How to do VASP convergence tests, and how to get two plots of Encut and k-points wrt to total energy? I want to do convergence for Mg2Si

I am using VASP for the first time and do not know how to do convergence tests. How are convergence tests done in VASP and how do we get two plots of Encut and k-points with respect to total energy? I ...
Shalini's user avatar
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5 votes
0 answers
158 views

At what threshold would I assure convergence? [closed]

I'm a beginner in DFT and Im using quantum espreso. I've seen the difference of hydrostatic pressure being used as a criteria for converging my k-point grid and plane-wave cut-off values. I would like ...
JasonDavis's user avatar
13 votes
1 answer
642 views

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

How much energy difference could be considered as tight convergence? Is total energy difference lower than 1 meV/atom good enough for kpoint convergence for total energy calculation?
Barix's user avatar
  • 231
6 votes
0 answers
159 views

Odd/even q-point mesh in harmonic phonon density of states (DoS) calculations [closed]

Total DoS for two different q-point grids, $100\times100\times100$ and $101\times101\times101$, are graphed above. The DoS for $63\times63\times63$, $65\times65\times65$, $71\times71\times71$, $72\...
Hitanshu Sachania's user avatar
6 votes
1 answer
1k views

Best practice(s) for q-points convergence in phonon density of states calculations

The answer can be a complete description of all the things we must be attentive to while converging q-points. A peculiar thing that I feel contradicted about: Generally, we don't take an equal # of k-...
Hitanshu Sachania's user avatar
7 votes
2 answers
101 views

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

Let's consider a fictitious AB type binary alloy. AB is known to exist in a B2 type ordered ...
Hitanshu Sachania's user avatar
12 votes
1 answer
1k views

Increasing supercell size to increase K-points sampling

Suppose I have a system with a property that is highly dependent on the density of k-points, namely the dielectric function. Using a small supercell, say with 4 atoms, I've reached the maximum allowed ...
Anibal Bezerra's user avatar
11 votes
2 answers
795 views

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

Smearing (smearing width $\sigma$, to be precise) has always been confusing. I understand what it is but can't pin it down when it comes to DFT calculations. Should it be converged like k-points and ...
Hitanshu Sachania's user avatar
9 votes
2 answers
338 views

k-points convergence for spin polarised vs non-polarised DFT calculations

Spin-polarised calculations come at a computational cost and I tend to take ISPIN = 2 only after I'm done with a proper initial relaxation of the structure, so the ...
Hitanshu Sachania's user avatar
22 votes
3 answers
4k views

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

Let's say I want to relax a structure using VASP. K-points and ENCUT convergence tests must be conducted before or after relaxation? and which tags should be ...
Chi Kou's user avatar
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13 votes
4 answers
3k views

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

Let's say I want to make k-point convergence test for graphite. And let's say it converges at 12x12x4. Do I first need to do a k-point convergence test for equal k-points on xyz directions then make ...
Taavi's user avatar
  • 131
16 votes
1 answer
678 views

Standard values for level of convergence

In matter modelling we use self consistent approaches for calculating different properties. Many DFT codes find the ground state of the system by determining total energy self consistently. Going ...
Thomas's user avatar
  • 9,092
23 votes
2 answers
423 views

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

As pointed out in a previous question each time you start a new DFT calculation, it is recommended to do a convergence test. If I am studying a system using different exchange correlation functional ...
Thomas's user avatar
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13 votes
2 answers
1k views

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

As I pointed out in the other question Convergence test I: same atoms and different structures, each time you start a DFT calculation you need to do a convergence test. Normally, we use the system ...
Camps's user avatar
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16 votes
2 answers
801 views

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?

Any time you start a new DFT calculation, it is recommended to do a convergence test. The parameters to include in the tests vary depending on the DFT implementation, i.e. plane-waves, Gaussian basis ...
Camps's user avatar
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