Questions tagged [convergence-test]

Questions relating to convergence testing methods in matter modeling.

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Variable cell relaxation method is not converging in Quantum ESPRESSO

I am trying to optimize the lattice constants 'a' and 'c' for a hexagonal lattice. I am using the PAW pseudopotential here. I have run the SCF calculation with ...
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7 votes
2 answers
320 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 ...
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10 votes
2 answers
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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 ...
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7 votes
1 answer
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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 ...
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6 votes
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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 ...
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7 votes
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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 ...
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13 votes
1 answer
328 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 ...
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14 votes
3 answers
509 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 ...
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10 votes
1 answer
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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, ...
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11 votes
1 answer
555 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 ...
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13 votes
3 answers
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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?
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5 votes
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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 ...
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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 ...
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13 votes
1 answer
289 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?
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6 votes
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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\...
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4 votes
1 answer
433 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-...
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7 votes
2 answers
79 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 ...
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11 votes
1 answer
542 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 ...
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11 votes
2 answers
257 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 ...
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9 votes
2 answers
186 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 ...
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18 votes
3 answers
2k 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 ...
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13 votes
4 answers
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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 ...
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15 votes
1 answer
253 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 ...
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21 votes
2 answers
241 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 ...
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13 votes
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489 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 ...
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16 votes
2 answers
284 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 ...
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