Questions tagged [numerical-convergence]

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

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The relationship between average eigenvalue and convergence performance in VASP?

I meet some convergence problems when doing self-consistent field calculation in VASP, I find some parameters on VASP wiki that might be able to tune the convergence performance. But I don't quite ...
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17 votes
4 answers
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What are the physical reasons if the SCF doesn't converge?

I've been mostly using ab initio methods as a routine calculation tool, so even though I know some things from the theoretical side, I still have mostly hands-on experience. One of the things that I ...
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8 votes
1 answer
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Use of higher-order integrators in molecular dynamics?

The usual verlet integration scheme for propagating molecular dynamics according to Newton's equations of motion looks like the following: $$ x(t+\Delta t)=2x(t)-x(t-\Delta t)+a(t)\Delta t^2 $$ This ...
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3 answers
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What are the advantages of the Davidson diagonalization method over other sparse matrix diagonalization methods?

I am interested to understand the advantages of the Davidson diagonalization method over other sparse matrix diagonalization routines. For instance, Intel MKL.
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7 votes
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How to make SCF converge in SIESTA?

I am trying to optimize some 4 atom Ni clusters. There are 4 types: one linear, one zigzag, one 2D and one 3D. Using the setup below, I don't get the calculations to converge even after 1000 SCF steps ...
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4 votes
0 answers
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DFT Vasp calculation not converging when doing a SCF calculation for monolayer MoS2 [closed]

I am trying to calculate the electronic structure of Monolayer Molybdenum Disulfide ($\ce{MoS_2}$) using Vasp. However, I encounter problems with convergence. It does not seem that my calculations ...
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7 votes
1 answer
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SCF Calculation not converging after two decimal places in Quantum ESPRESSO

I have been trying to do geometrical optimization for ZnSe Quantum Dots using Quantum ESPRESSO. The convergence is very slow, so slow that even after 200 iterations I could not get convergence. I ...
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10 votes
1 answer
329 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: ...
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7 votes
1 answer
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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 ...
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14 votes
1 answer
294 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. My topic of interest is computational materials science. So, one ...
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7 votes
1 answer
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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 ...
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11 votes
1 answer
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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 ...
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12 votes
1 answer
433 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 ...
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8 votes
1 answer
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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: ...
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11 votes
1 answer
156 views

Are there any other options beyond increasing the k-sampling and the number of unoccupied bands, to achieve better results?

I am trying to determine the optical properties of bulk Aluminium and related alloys using Quantum ESPRESSO. I am using both the epsilon.x and the simple.x modules to get wavelength-dependent ...
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11 votes
1 answer
137 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\...
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7 votes
2 answers
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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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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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8 votes
0 answers
147 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 ...
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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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11 votes
2 answers
512 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 ...
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26 votes
3 answers
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 ...
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12 votes
2 answers
182 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 ...
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14 votes
2 answers
245 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 ...
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40 votes
4 answers
2k 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, ...
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21 votes
4 answers
1k 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 ...
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