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Constrained DFT and photodoping

Constrained density functional theory (cDFT) is an extension of standard DFT in which one imposes explicit constraints - such as fixed charges, spins, or orbital occupations - on selected regions or states of a system via Lagrange multipliers. By enforcing these conditions, cDFT enables the direct calculation of localized excited states, charge‐transfer energies, redox potentials, and reaction barriers that are difficult to access with unconstrained ground‐state DFT.

Recently, it was proposed in Phys. Rev. B 104, 144103 (2021) to simulate the behavior of photoexcited insulators from cDFT, by imposing the presence of an excited, thermalized electron-hole plasma. This is in the spirit of a two-temperature model. Details of the approach can be found in the original paper.

In order to investigate the change of electronic structure in the presence of photo-electrons, or to perform structural optimization and get the variation of the lattice parameter versus the number photoexcited carriers, one needs to specify how to set the electron-hole plasma.

Below is an input example. 


  SmearingFunction = cold_smearing
  Smearing = 0.136*eV

  PhotoDopingBand = 5
  PhotodopingSmearing = 0.136*eV
  PhotoDopingNumElectrons = 0.1

In this example, the first two variables are used to define the smearing of the occupations, as one would usually do for a metallic system.

Now, in order to describe a photoexcited insulator, other variables need to be employed: