# DFT: Investigating Change in States at Surfaces and Interfaces

• Arcturus82
In summary, the conversation discusses the use of Kohn-Sham eigenvalues in Density Functional Theory (DFT) to construct the band structure and density of states (DOS). It is mentioned that for a 3D extended system, the eigenvalues are determined up to a constant and for a 2D extended surface, a well-defined energy reference exists. The conversation then shifts to the investigation of state changes when creating a surface from a bulk system and an interface from two free surfaces. It is suggested that in order to compare the states, the same energy reference must be used for both systems. For the surface system, the electrostatic potential can be evaluated and the vacuum level aligned with the DOS at the surface layer. However, for
Arcturus82
Dear all,

In Density Functional Theory (DFT) the Kohn-Sham eigenvalues are used to construct the band structure and the density of states (DOS). For a 3D extended system the eigenvalues are determined up to a constant since there is no absolute energy reference, while for a 2D extended surface a well-defined energy reference exists (the vacuum level).

I now want to look at

a) Which states that changes (i.e. the energy range) when creating a surface from a bulk system (e.g. to find surface resonances).

b) Which states that changes when creating an interface from two free surfaces.

In order to investigate the change in states I have to have the same energy reference for both systems.

For a) I can for the surface system evaluate the electrostatic (and the exhange-correlation) potential and align my vacuum level with my DOS at the surface layer. Then if my slab is sufficiently thick I can also align the bulk DOS.

How can I proceed for case b) ? For each respective surface calculation I can relate the eigen values to the vacuum level, but for the interface calculation I run into to difficulties since I don't have an absolute reference. As far as I can see it the Fermi levels for the surfaces and the interface can't be aligned w.r.t. each other to due the lack of this absolute reference. Is my conclusion correct or can I still align them at the Fermi level? If the latter is valid, why is it so?

I think that as long as you don't shift your states so that the Fermi level is at zero then you will be able to properly compare them directly. The eigenvalues are determined up to an additive constant, but that constant will be present in your Hamiltonian. Your potential in either case is $$V(r) = \sum_i -Ze/r_i$$ where i runs over the nuclear positions. Since you don't have an extra constant that is changed from one calculation to another both will give you energies relative to the same reference.

## What is DFT and how is it used in surface and interface investigations?

DFT stands for Density Functional Theory, and it is a computational method used to study the electronic structure and properties of materials. It is commonly used in surface and interface investigations to understand the behavior and changes of matter at these boundaries.

## What types of changes can be investigated using DFT at surfaces and interfaces?

DFT can be used to investigate a variety of changes at surfaces and interfaces, including structural changes, chemical reactions, and electronic properties. It can also provide insights into the adsorption and desorption of molecules on surfaces.

## How does DFT compare to other methods of studying surfaces and interfaces?

DFT is a powerful method for studying surfaces and interfaces because it takes into account the interactions between electrons and nuclei, which are crucial for understanding the behavior of matter at these boundaries. Other methods, such as classical molecular dynamics, do not consider these interactions and therefore may not provide as accurate results.

## What are the limitations of DFT in investigating surfaces and interfaces?

While DFT is a powerful method, it does have some limitations. It is most accurate for studying systems with weak interactions, such as van der Waals forces. It also requires a good understanding of the system being studied and may not be suitable for complex systems with strong interactions.

## How can DFT be used to design and improve materials for surface and interface applications?

DFT can aid in the design and improvement of materials for surface and interface applications by providing a detailed understanding of the electronic and structural changes that occur at these boundaries. This information can be used to predict and optimize the properties of materials for specific applications, such as catalysis or energy storage.

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