Questions tagged [numerical-convergence]

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

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VASP: Meaning of Certain Output Tags

This is just a quick question about the output tags in VASP. My output file is of the form: ...
frobenius's user avatar
1 vote
0 answers
29 views

LBFGS -- Incomplete Convergence due to BadIntpln [closed]

I am doing energy minimization of a molecular system in TINKER. While doing so, I am getting the error in the title of this question. What might be my problem?
Himanshu's user avatar
4 votes
2 answers
258 views

Can the the relaxed energy of a material system (eV/atom) computed by DFT using VASP be positive?

If it is positive, why is it so? Is it due to specific convergence criteria or choice of pseudopotentials? I cannot check the convergence of the DFT calculations as they have been downloaded from a ...
Pranoy Ray's user avatar
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9 votes
1 answer
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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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Convergence issues in DFTB+ calculations?

I am trying to calculate the Fermi level of a DNA-graphene system. I was able to get the calculations for graphene and DNA strand to converge separately. However, when I try to calculate the same for ...
Hemanth Haridas's user avatar
4 votes
2 answers
185 views

Dirac exchange energy and X_alpha LDA methods

I want to make a DFT code (by myself) that works in the RKS LDA H2 configuration. This post is a continuation of here. Now by adding the Vosko, Wilk, Nusair correlation I have much better results : ...
mle's user avatar
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6 votes
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SCF energy keeps on fluctuating between two values, instead of converging, why?

Sometimes I have observed this phenomenon where my calculations' SCF iterations keep on fluctuating between two values, and not converging. I have observed it in all sort of systems, ranging from ...
ipcamit's user avatar
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6 votes
1 answer
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Total number of electrons from DOS and PDOS - what's the theory behind it?

Let say we need to determine the total number of electrons of an FCC material (let say, two silicon atoms in the unit cell, using numerical/mathematical approach. How can we get the total number of ...
Sak's user avatar
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3 votes
1 answer
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Electronic Convergence Issues with Nonlocal vdW-DF Functionals (in VASP)

Compared to say the DFT-D3 type of dispersion corrections (set with IVDW in VASP), I've found the nonlocal vdW-DF functionals (described here in the VASP Wiki) to be much more difficult to converge ...
CW Tan's user avatar
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9 votes
1 answer
605 views

How does charge mixing work?

When doing DFT calculations on certain magnetic materials, convergence can be difficult. This problem can be solved by changing the mixing parameters (e.g. AMIN and BMIX in the INCAR file of VASP). My ...
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7 votes
2 answers
525 views

What are the limitations of FCI?

I am aware of the fact that Full Configuration Interaction scales poorly. Taking into account that the number of $n$-times excited Slater determinants of an $N$-particle System with $M$ basis ...
Jannis Erhard's user avatar
4 votes
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How to perform a DFT/def2-PPCBS extrapolation on NWChem [closed]

I'm planning to analyse an extremely complex molecule on NWChem(it's practically the only free computational package available package here, and the molecule is so large that running a seemingly ...
Kanghun Kim's user avatar
7 votes
1 answer
599 views

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 ...
Jack's user avatar
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19 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 ...
user avatar
8 votes
1 answer
203 views

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.
Wychh's user avatar
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8 votes
1 answer
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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 ...
Camps's user avatar
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4 votes
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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 ...
PHy's user avatar
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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 ...
Parmeet Singh EP 066's user avatar
10 votes
1 answer
1k 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: ...
Binh Thien's user avatar
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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 ...
ksousa's user avatar
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14 votes
1 answer
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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 ...
Jonathan's user avatar
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7 votes
1 answer
268 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 ...
Dr.Viper's user avatar
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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 ...
Tristan Maxson's user avatar
12 votes
1 answer
493 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 ...
jw_'s user avatar
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8 votes
1 answer
594 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: ...
cp2kuser's user avatar
11 votes
1 answer
231 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 ...
Anibal Bezerra's user avatar
11 votes
1 answer
212 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\...
Xiaoming Wang's user avatar
7 votes
2 answers
100 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
9 votes
2 answers
328 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
8 votes
0 answers
259 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 ...
Alfred's user avatar
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15 votes
1 answer
635 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
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12 votes
2 answers
1k 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 ...
bad_chemist's user avatar
29 votes
3 answers
3k 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 ...
Alfred's user avatar
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12 votes
2 answers
259 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 ...
Paulie Bao's user avatar
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15 votes
2 answers
577 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 ...
Tyberius's user avatar
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41 votes
4 answers
7k 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, ...
BernhardWebstudio's user avatar
22 votes
4 answers
2k 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 ...
Michael F. Herbst's user avatar