Density Functional Theory and pseudopotentials

In summary, the conversation discusses the author's blog on computational physics, specifically their recent projects on DFT with plane waves basis and the 'supercell' method for molecules. The author initially tried to avoid using pseudopotentials for a 'toy' program, but found that they were necessary for accurate results. They are currently searching for local pseudopotentials for more atoms and have found some options from Sandia, but are hesitant to use them due to added complexity.
  • #1
aaroman
55
15
Hi,

I have a blog oriented on computational physics: https://compphys.go.ro For many posts I have a GitHub project. Lately I started some DFT oriented ones, the latest being a DFT (with plane waves basis) project for a 'quantum dot'.

Currently I started working on a project that will use the 'supercell' method for molecules, something very similar with the Hartree-Fock project also described on the blog.
I thought that I could go away without using pseudopotentials, for such a 'toy' program, but I was very wrong. I tried computing the H2O molecule and the results are very disappointing. I have to go to 128x128x128 grid to get close to results I obtain with STO3G orbitals set from my Hartree-Fock project, and computation takes a lot of time, compared with HF (a fraction of a second for the later).

I understand why, although I did not expect such big differences, but anyway, apparently I need pseudopotentials. I would like to avoid for now the additional complexity of non local pseudo potentials. Adding a local one seems quite easy, the only problem is I cannot find such pseudopotentials for enough atoms. I found this: https://carter.princeton.edu/research/local-pseudopotentials/ and that's about it.

Does anybody know a way to get local pseudopotentials for more atoms?

Thank you!
 
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  • #3
After a quick look, they all seem to generate pseudopotentials that are not local (despite having a local part). That adds complexity to the DFT code, I'm not sure I want to deal with that for now. I'll see how it works with the local potentials I found and if it works, I might think about adding code dealing with projectors and so on...
 

FAQ: Density Functional Theory and pseudopotentials

What is Density Functional Theory (DFT)?

Density Functional Theory (DFT) is a computational method used to study the electronic structure of molecules, solids, and liquids. It is based on the concept of the electron density, which is a more fundamental quantity than the wave function used in traditional quantum mechanical calculations. DFT provides a cost-effective approach for predicting the properties of materials and has a wide range of applications in chemistry, physics, and materials science.

How does DFT differ from other quantum mechanical methods?

DFT differs from other quantum mechanical methods in that it uses the electron density instead of the wave function to describe the electronic structure of a system. This makes DFT more computationally efficient and allows for the study of larger and more complex systems. However, DFT is based on approximations and does not provide exact solutions like other methods such as Hartree-Fock theory.

What are pseudopotentials and why are they used in DFT?

Pseudopotentials are effective potentials that are used to replace the core electrons in a system when performing DFT calculations. This is because the core electrons do not contribute significantly to the bonding and properties of a material, and including them in the calculations would make them much more computationally intensive. Pseudopotentials allow for faster and more accurate DFT calculations by reducing the number of electrons that need to be considered.

How are pseudopotentials generated?

Pseudopotentials are generated using a process called pseudopotential construction. This involves fitting the radial part of the potential for the core electrons to reproduce the same energy levels and electron density as the all-electron potential. The pseudopotentials can then be optimized for different levels of accuracy and for different elements in the periodic table.

What are the limitations of DFT and pseudopotentials?

Although DFT is a powerful and versatile method, it has some limitations. One major limitation is that it relies on approximations, which means that the results may not be entirely accurate. Additionally, pseudopotentials may not accurately capture the behavior of highly localized electrons and can lead to errors in certain types of calculations. Care must also be taken in choosing the appropriate pseudopotentials for a specific system to ensure accurate results.

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