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 ...
- 33.6k
20
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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20
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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18
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 ...
- 546
15
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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15
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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13
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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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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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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10
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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10
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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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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9
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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 ...
- 18k
9
votes
Accepted
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 ...
- 1,416
9
votes
Accepted
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 ...
- 2,561
8
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 ...
- 1,643
8
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 ...
- 1,247
8
votes
Accepted
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. ...
- 1,416
8
votes
Accepted
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
Accepted
Effective core potential (ECP) calculations
This is perfectly normal.
In ab initio calculations, the energy zero point is usually chosen as the state where all explicitly described particles are infinitely far from each other and each ...
- 9,076
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 $...
- 459
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 ...
- 33.6k
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 ...
- 1,247
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 ...
- 18k
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 ...
- 33.6k
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 ...
- 361
6
votes
Accepted
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 ...
- 33.6k
6
votes
Understanding Illustration for Pseudopotential
These are quite a few questions and it is difficult to grab the essence of what you actually want to know, but maybe I can clarify a few aspects that help to understand these things.
Besides the ...
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