27 votes
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What is a Padé approximant?

To complement mykd's already excellent answer, I will just add that the approximant we all learn in school (the Taylor approximant) is nice to teach and easy to help students learn the concept of ...
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  • 28.6k
19 votes

What is a Padé approximant?

PADE refers to the mathematician Henri Padé, who developed the Padé approximation, which can approximate a function using rational polynomial functions. For ex: $$ \sin(x)\approx {\frac {(12671/...
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  • 1,982
17 votes
Accepted

Linearized Augmented Plane Wave Method vs. Pseudopotential Methods

Disclaimer: I am a developer of the Fleur code. I hope that I don't put too much bias into this answer. At least I try to not do that... When you want to solve the Kohn-Sham equations you have the ...
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16 votes

Norm-Conserving or Ultrasoft, which pseudopotential to choose?

Three criteria to consider are Performance and size of basis set USPPs generally require a lower plane-wave cut-off and smaller basis, but a larger density grid. However performance may not be ...
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14 votes
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When do relativistic effects need to be explicitly included?

When to include relativistic corrections or modeling of any kind in computational methods is a rather complex one. Full Dirac methods as you asked about (DHF) recapture two important factors, so ...
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12 votes
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Understanding the GTH Pseudopotential

Great question, will allow me to advertize our newly-released DFT code :](https://github.com/JuliaMolSim/DFTK.jl/). The rloc parameter is the characteristic distance at which the pseudopotential acts....
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12 votes

What are the types of pseudopotentials?

Summary of the "milestone" pseudopotential (PP) papers Since it wasn't available anywhere & took me a few hours, it's an answer rather than question-edit: Local pseudo-potentials: 1935: ...
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12 votes
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When are atomic-orbital-basis (rather than plane-wave) methods appropriate in periodic DFT?

The clearest example in my mind is if you want to understand the orbital-based contributions to some phenomena (e.g. bonding, a reaction energy), particularly if the periodic material being modeled is ...
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  • 6,942
11 votes

What is the difference between Ultrasoft, ONCV and PAW Pseudopotentials? Which is better for a spin-orbit coupled calculation?

Pseudopotentials (PPs) describe the effective interaction between the valence electrons and a nuclei screened by frozen core electrons. This approximation makes DFT calculations less computationally ...
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  • 358
10 votes

How to convert Unified Pseudopotential Format (UPF) into Ultrasoft Pseudopotential (USPP) format?

USPP is not a format of pseudopotential. It is a type of pseudopotential that enables you to work with lower cutoffs. On the other hand, upf is a popular file extension for pseudopotentials. If your ...
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  • 2,212
9 votes

Are meta-GGA functionals SCAN (and its variants) ok to use in planewave DFT with the PBE pseudopotentials?

It is well known that using pseudopotentials generated with a functional which is inconsistent with the functional used in the plane-wave calculation can lead to serious problems, see here for example....
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  • 191
9 votes
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Why are Norm-conserving Pseudopotentials considered so important?

I think the best place to start is the original paper¹ proposing norm conserving pseudopotentials (NCPPs). It's very short and gives a nice explanation of why they were developed. I'll just give a ...
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  • 14k
9 votes

For which elements should I include spin orbit coupling in Quantum ESPRESSO?

Spin-orbit coupling is related to relativity which is increasingly important for heavier elements. If you're aiming for ultra-high precision, as in this post on our site: How accurate are the most ...
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9 votes

When should I include semi-core electrons in DFT calculations?

I am quoting from the book Materials Modelling Using Density Functional Theory: Properties and Predictions by Feliciano Giustino How do we decide which wavefunctions should be considered ‘core’ and ...
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  • 8,402
9 votes
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Error in nscf calculation while considering Spin Orbit Coupling in Quantum ESPRESSO

This error has been resolved now. Though I am not an expert but here are few thoughts. There may be several reasons for this error: This error might appear due to numerical instability from ...
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  • 2,361
8 votes
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Can GW-BSE quasiparticle calculations only be run on norm-conserving pseudopotentials (NCPPs)? If so, why not?

The GPAW code can do GW/BSE and it uses purely PAW potentials. So it's absolutely possible to do GW/BSE with PAW sets. The involvement for the implementation, however, is another topic. From Kevin J. ...
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8 votes
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For which elements should I include spin orbit coupling in Quantum ESPRESSO?

But in the case of the pseudo-potentials included, should I use fully relativistic pseudopotentials for all three elements? Yes, you should use the fully relativistic pseudopotentials if you are ...
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8 votes
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VASP GW PAW Pseudopotentials

A lot of it will be in the difference in cost. But as a rule of thumb, GW potentials are typically harder, and include more states (semicores). This need not always be the case, since if you look at ...
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7 votes

Analytical expression for an atomic lattice "muffin-tin" potential for purposes of illustration and simple scattering calculations

The Augmented Plane Wave (APW) method, and by extension Linearly-Augmented Plane Wave method are both generalizations of the Muffin Tin Approximation. In both the APW and LAPW methods, the potential $...
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  • 439
7 votes

Are there properties that can only be calculated with a given set-up?

Only a partial answer: some setups are surely better suited than others for a given property. Some developers recognize this and thus do not bother implementing properties for which their setup will ...
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  • 1,603
7 votes

What are the types of pseudopotentials?

To add to Nike's list, one should also differentiate between the energy consistent pseudopotentials used in quantum chemistry and the shape consistent pseudopotentials that dominate in the solid state ...
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7 votes

Norm-Conserving or Ultrasoft, which pseudopotential to choose?

The short answer is DEPENDS on your system and property of interest you are after (among other things). Anyway, I see the order of physical basis/complexity increasing as NC --> USPP --> PAW. But I ...
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  • 1,237
6 votes

VASP Dielectric constants: What potential to use?

I don't think there is a consensus of norm conserving PP being more accurate. There are some references I am aware of which have calculated dielectric tensor using NC-PP, but without justification ...
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  • 1,237
6 votes

How to generate a pseudopotential for a cation/anion?

Why would you want to generate a pseudopotential for a cation/anion? The pseudopotential is only for the inner core electrons. The only case where you'd want to do this is when the charge state of the ...
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6 votes
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What is the meaning of energy-consistent and shape-consistent in the context of pseudopotentials?

This 2017 paper titled "Shape and Energy Consistent Pseudopotentials for Correlated Electron systems" defines energy-consistent psuedopotentials this way: "A combined reproduction of ...
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6 votes

What are some of the advantages and disadvantages when using shape consistent pseudopotentials as compared to energy consistent pseduopotentials

The advantages and disadvantages are discussed in this review paper which was cited in Susi Lehtola's answer to: What are the types of pseudopotentials? and then again in my answr to: What is the ...
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6 votes

Recommendations to treat 4f electrons (valence) of Yb3+ in magnetic systems in VASP

There is a discussion on this limitation on VASP page. Maybe the approach of a frozen f valence PAW pseudopotential could help you. Although magnetic properties might not be as well represented. I'm ...
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6 votes
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Citing a pseudopotential

Other answers are most welcome, but here I will outline the process through which I went to find a reference that you can cite for a hydrogen pseudopotential. Hopefully this will help you find ...
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6 votes
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Can energies obtained with different pseudopotentials be compared?

As a first approximation, the total energy will depend on the number of electrons in valence for a given pseudo -potential. This should explain the huge difference in between the two set of ...
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5 votes

When are atomic-orbital-basis (rather than plane-wave) methods appropriate in periodic DFT?

The question is related essentially to solve the Kohn-Sham equation with an atomic-like basis set. In fact, different implementations of DFT are distinguished mainly by their basis set and how they ...
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