Density functional theory and partial charge transfer

In summary, when using density functional theory to simulate the interaction between a molecule and a surface, there are two types of interactions: covalent bond formation and charge-transfer complexes. The presence of localized orbitals shared between the molecule and the substrate indicates the presence of covalent bonding. However, molecules can also be bound through other mechanisms, such as static or dynamical electric interactions, which would be classified as purely physical with no chemical bonding component. The determination of whether a covalent bond has formed can be done by localizing the occupied orbitals of the Kohn-Sham determinant and analyzing the results.
  • #1
arrektor
15
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When density functional theory is used to simulate a molecule adsorbed on a surface, it turns out that due to their interaction, a fraction of an electron is transferred from the surface to the molecule or vice versa.

These interactions are normally categorised in interactions involving covalent bond formation (also termed as chemisorption), or charge-transfer complexes (also termed as physisorption).

The situation where the charge transfer is integer,i.e., the molecule is in an anionic or cationic state doesn't normally exist.

I was wondering what is the physical meaning of this and how one could determine by the nature of these interactions whether there is a chemical bond being formed or whether it is just physisorption of the molecule on the surface?
 
  • #3
I would say that the question of whether or not a (partial) covalent bond has been formed can be decided by localizing the occupied orbitals of the Kohn-Sham determinant and analyzing the result. If the localized orbitals can be split exactly into two categories: One set of occupied orbitals only on the molecule, and one set only on the substrate, then no covalent bonding is present. However, if some localized orbitals have substantial contributions on both sides[1], then a covalent character exists. Note that since occupied orbitals have integer occupations, a non-integer total charge on the molecule does imply the presence of localized orbitals which are shared between the molecule and the substrate, and thus the presence of some (potentially small) degree of covalent bonding.

However, charge transfer is not the only mechanism by which molecules could be bound. For example, it is entirely possible that the number of electrons on the molecule is integer, that there is no covalent bonding, and that a physisorption results from either static electric interactions (e.g., molecular dipole moments resulting from the actual distribution the integer of charge across the molecule and the surface) or dynamical electric interactions (i.e., London dispersive interactions). These kinds of van der Waals interactions would typically be classified as purely physical in nature with no chemical bonding component.

[1] This interpretation depends on a definition of partial charge, of course, which is not 100% physically observable (although various reasonable definitions have been given, e.g., http://dx.doi.org/10.1002/jcc.10351 or http://dx.doi.org/10.1021/ct400687b )
 

1. What is density functional theory (DFT)?

Density functional theory is a computational method used in the field of quantum mechanics to study the electronic structure and properties of molecules and materials. It is based on the idea that the total energy of a system can be determined by the electron density rather than the wave function of the system.

2. How does DFT calculate partial charge transfer?

DFT calculates partial charge transfer by using the electron density of the system and evaluating the change in electron density between two distinct states of the system. This change in electron density corresponds to the partial charge transfer between the two states.

3. What are the advantages of using DFT in studying partial charge transfer?

DFT is relatively fast and accurate compared to other methods of studying partial charge transfer. It also takes into account the effects of electron-electron interactions and allows for the inclusion of more complex systems, making it a versatile and widely used method in computational chemistry.

4. Are there any limitations to DFT in studying partial charge transfer?

Yes, there are some limitations to DFT in studying partial charge transfer. It may not accurately capture certain phenomena such as dispersion forces and charge delocalization. It also relies on approximations and can be sensitive to the choice of exchange-correlation functional used.

5. How is DFT used in practical applications involving partial charge transfer?

DFT is used in a wide range of practical applications, including drug design, materials science, and catalysis. It can provide valuable insights into the electron transfer processes that occur in these systems, aiding in the development of new and improved materials and processes.

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