Questions tagged [numerical-convergence]

For questions about numerical convergence in molecular and materials modeling questions.

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10
votes
1answer
80 views

Why does the VASP electronic step not stop after getting converged?

I have just implemented VASP code with Gamma K-point, POSCAR of 128-atom supercell and INCAR like this: ...
6
votes
1answer
88 views

Is it possible to get complex numbers as solutions for a secular determinant in simple Hückel method (SHM)?

Past semester we revisited the Hückel molecular orbital theory at class. One day I was trying to solve some problems with SymPy, a Python module that is a computer algebra system (CAS), and noticed ...
13
votes
1answer
221 views

DFT Knowledge Check for Posed Problem

Currently I am trying to apply to a graduate program overseas. They require me to include a research proposal in my application. This doesn't make much sense to me since how can I write a research ...
7
votes
1answer
107 views

Smearing and charge balance for tungsten with interstitial oxygen in Quantum ESPRESSO

What smearing would be ideal for a bcc W with interstitial O atom. I am running 3x3x3 unit cells with one N interstitial. I am having convergence issues in relax ...
9
votes
0answers
58 views

Charge density convergence within VASP using spin polarized DFT+U

I am noticing a possible convergence problem in a magnetic U system I am studying. In the OSZICAR file I often see the rms(c) value go to about 0.2e+3 and stabilize. Can anyone suggest what may be ...
12
votes
1answer
399 views

Are there examples of ab initio predictions on small molecules without the “major approximations”?

In most textbooks the precise Schrödinger euqation of a molecule is given (and the "beautiful thing" stops here), then born oppenheimer approximation is made, then layers of other ...
8
votes
1answer
65 views

Failure of CP2K density functional software to converge for a 64 Si atom amorphous structure

The structure was created using Stillinger Weber with LAMMPS. The command I used was the following: ...
11
votes
1answer
84 views

adaptive k-mesh

Are there any known codes or scripts to generate an adaptive k-mesh for DFT calculations? For example, I'd like to make the k-mesh near the $\Gamma$ point (e.g., half the zone) more denser ($20\...
7
votes
2answers
68 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 ...
8
votes
2answers
110 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 ...
7
votes
0answers
92 views

ASE's fmax and FHI-AIMS's relax_geometry bfgs “tolerance”. Are they same? [closed]

They look like the same thing, but their definitions are confusing. Additionally, while it's possible to use 'tolerance'< 0.05 in AIMS easily, in ASE, some structures never optimize if you choose ...
14
votes
1answer
185 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 ...
11
votes
2answers
200 views

Calculating diffusion coefficient from Mean Squared Displacement

I am performing a molecular dynamics simulation and I have calculated the velocity autocorrelation function (VACF), $\psi (t)$ and mean square displacement, $\langle (\Delta r)^2 \rangle$. In my ...
25
votes
3answers
2k views

How big should a supercell be in phonon calculations?

Is 2x2x2 enough, or is larger needed? I know that a convergence test must be carried out, but increasing supercell size enormously increases computational time, and I am using the Python program ...
11
votes
2answers
153 views

What is the mathematical condition that ensure that the self-consistent field (SCF) procedure must converge?

We know that there is convergence issue of DFT method on study real molecular system. It would be conducive if people could pre-determine if the SCF procedure is converge analytically before starting ...
14
votes
2answers
138 views

Convergence by energy and/or density?

The SCF is an iterative procedure to produce the ground state energy and wavefunction for Hartree-Fock, MCSCF, and DFT. Due to the iteration, one needs to establish convergence criteria. Depending on ...
35
votes
2answers
355 views

How should one choose the time step in a molecular dynamics integration?

Choosing too small of a timestep leads to an unrealistic simulation time, whereas too big of a timestep leads to the system not being represented correctly (or, in the case of an algorithm like SHAKE, ...
20
votes
4answers
490 views

Difficult cases for converging Kohn-Sham SCF calculations

In a recent article by Woods et. al.[1] a couple of methods for converging ground-state Kohn-Sham DFT calculations are reviewed and compared. In their test suite they give plenty of examples for badly ...