Local-density approximation: Difference between revisions

Revision as of 10:07, 27 May 2022

Revision as of 01:28, 31 October 2022

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== Applications ==

Local density approximations, as with GGAs are employed extensively by [[solid-state physics|solid state physicists]] in ab-initio DFT studies to interpret electronic and magnetic interactions in semiconductor materials including semiconducting oxides and [[spintronics]]. The importance of these computational studies stems from the system complexities which bring about high sensitivity to synthesis parameters necessitating first-principles based analysis. The prediction of [[Fermi level]] and band structure in doped semiconducting oxides is often carried out using LDA incorporated into simulation packages such as CASTEP and DMol3.<ref>{{cite journal| last1=Segall| first1=M.D.| last2=Lindan| first2=P.J | title= First-principles simulation: ideas, illustrations and the CASTEP code | journal= Journal of Physics: Condensed Matter | year= 2002| volume=14| issue=11| pages=2717|bibcode = 2002JPCM...14.2717S |doi = 10.1088/0953-8984/14/11/301 | s2cid=250828366}}</ref> However an underestimation in [[Band gap]] values often associated with LDA and [[Density functional theory#Approximations .28exchange-correlation functionals.29|GGA]] approximations may lead to false predictions of impurity mediated conductivity and/or carrier mediated magnetism in such systems.<ref>{{cite journal| last1=Assadi| first1=M.H.N| title= Theoretical study on copper's energetics and magnetism in TiO₂ polymorphs| journal= Journal of Applied Physics | year=2013| volume=113| issue=23| pages= 233913–233913–5| doi=10.1063/1.4811539|arxiv = 1304.1854 |bibcode = 2013JAP...113w3913A | s2cid=94599250|display-authors=etal}}</ref> Starting in 1998, the application of the [[Rayleigh theorem for eigenvalues]] has led to mostly accurate, calculated band gaps of materials, using LDA potentials.<ref>{{Cite journal|last1=Zhao|first1=G. L.|last2=Bagayoko|first2=D.|last3=Williams|first3=T. D.|date=1999-07-15|title=Local-density-approximation prediction of electronic properties of GaN, Si, C, and RuO2|journal=Physical Review B|volume=60|issue=3|pages=1563–1572|doi=10.1103/physrevb.60.1563|bibcode=1999PhRvB..60.1563Z |issn=0163-1829}}</ref><ref>{{Cite journal|last=Bagayoko|first=Diola|date=December 2014|title=Understanding density functional theory (DFT) and completing it in practice|journal=AIP Advances|volume=4|issue=12|pages=127104|doi=10.1063/1.4903408|bibcode=2014AIPA....4l7104B |issn=2158-3226|doi-access=free}}</ref> A misunderstanding of the second theorem of DFT appears to explain most of the underestimation of band gap by LDA and GGA calculations, as explained in the description of [[density functional theory]], in connection with the statements of the two theorems of DFT.

== Homogeneous electron gas ==

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:<math>\epsilon_{\rm c} = \frac{1}{2}\left(\frac{g_{0}}{r_{\rm s}} + \frac{g_{1}}{r_{\rm s}^{3/2}} + \dots\right)\ ,</math>

where the [[Wigner–Seitz cell|Wigner-Seitz parameter]] <math>r_{\rm s}</math> is dimensionless.<ref name="Murray Gell-Mann and Keith A. Brueckner 1957 364">{{cite journal | title = Correlation Energy of an Electron Gas at High Density | author = Murray Gell-Mann and Keith A. Brueckner | journal = Phys. Rev. | volume = 106 | pages = 364–368 | year = 1957 | doi = 10.1103/PhysRev.106.364 | issue = 2| bibcode = 1957PhRv..106..364G | s2cid = 120701027 | url = https://authors.library.caltech.edu/3713/1/GELpr57b.pdf }}</ref> It is defined as the radius of a sphere which encompasses exactly one electron, divided by the Bohr radius. The Wigner-Seitz parameter <math>r_{\rm s}</math> is related to the density as

:<math>\frac{4}{3}\pi r_{\rm s}^{3} = \frac{1}{\rho}\ .</math>

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An analytical expression for the full range of densities has been proposed based on the many-body perturbation theory. The calculated correlation energies are in agreement with the results from [[quantum Monte Carlo]] simulation to within 2 milli-Hartree.

Accurate [[quantum Monte Carlo]] simulations for the energy of the HEG have been performed for several intermediate values of the density, in turn providing accurate values of the correlation energy density.<ref>{{cite journal | title = Ground State of the Electron Gas by a Stochastic Method | author = D. M. Ceperley and B. J. Alder | journal = Phys. Rev. Lett. | volume = 45 | pages = 566–569 | year = 1980 | doi = 10.1103/PhysRevLett.45.566 | bibcode=1980PhRvL..45..566C | issue = 7| s2cid = 55620379 | url = https://digital.library.unt.edu/ark:/67531/metadc1059358/ }}</ref>

== Spin polarization ==