LDAUTYPE: Difference between revisions

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Description: {{TAG|LDAUTYPE}} specifies which type of L(S)DA+U approach will be used.
Description: {{TAG|LDAUTYPE}} specifies which type of L(S)DA+U approach will be used.
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The L(S)DA often fails to describe systems with localized (strongly correlated) ''d'' and ''f''-electrons (this manifests itself primarily in the form of unrealistic one-electron energies). In some cases this can be remedied by introducing a strong intra-atomic interaction in a (screened) Hartree-Fock like manner, as an on site replacement of the L(S)DA. This approach is commonly known as the L(S)DA+U method. Setting {{TAG|LDAU}}=.TRUE. in the {{FILE|INCAR}} file switches on the L(S)DA+U.
*{{TAG|LDAUTYPE}}=1: The rotationally invariant LSDA+U introduced by Liechtenstein ''et al.''<ref name="liechtenstein:prb:95"/>
*{{TAG|LDAUTYPE}}=1: The rotationally invariant LSDA+U introduced by Liechtenstein ''et al.''<ref name="liechtenstein:prb:95"/>
:This particular flavour of LSDA+U is of the form
:This particular flavour of LSDA+U is of the form

Revision as of 11:51, 2 March 2011

LDAUTYPE = 1 | 2 | 4
Default: LDAUTYPE = 2 

Description: LDAUTYPE specifies which type of L(S)DA+U approach will be used.


The L(S)DA often fails to describe systems with localized (strongly correlated) d and f-electrons (this manifests itself primarily in the form of unrealistic one-electron energies). In some cases this can be remedied by introducing a strong intra-atomic interaction in a (screened) Hartree-Fock like manner, as an on site replacement of the L(S)DA. This approach is commonly known as the L(S)DA+U method. Setting LDAU=.TRUE. in the INCAR file switches on the L(S)DA+U.

  • LDAUTYPE=1: The rotationally invariant LSDA+U introduced by Liechtenstein et al.[1]
This particular flavour of LSDA+U is of the form
and is determined by the PAW on-site occupancies
and the (unscreened) on-site electron-electron interaction
where |m⟩ are real spherical harmonics of angular momentum L=LDAUL.
The unscreened e-e interaction Uγ1γ3γ2γ4 can be written in terms of the Slater integrals , , , and (f-electrons). Using values for the Slater integrals calculated from atomic orbitals, however, would lead to a large overestimation of the true e-e interaction, since in solids the Coulomb interaction is screened (especially ).
In practice these integrals are therefore often treated as parameters, i.e., adjusted to reach agreement with experiment in some sense: equilibrium volume, magnetic moment, band gap, structure. They are normally specified in terms of the effective on-site Coulomb- and exchange parameters, U and J (LDAUU and LDAUJ, respectively). U and J are sometimes extracted from constrained-LSDA calculations.
These translate into values for the Slater integrals in the following way (as implemented in VASP at the moment):
- -
-
The essence of the LSDA+U method consists of the assumption that one may now write the total energy as:
where the Hartree-Fock like interaction replaces the LSDA on site due to the fact that one subtracts a double counting energy , which supposedly equals the on-site LSDA contribution to the total energy,
  • LDAUTYPE=2: The simplified (rotationally invariant) approach to the LSDA+U, introduced by Dudarev et al.[2]
This flavour of LSDA+U is of the following form:
This can be understood as adding a penalty functional to the LSDA total energy expression that forces the on-site occupancy matrix in the direction of idempotency,
.
Real matrices are only idempotent when their eigenvalues are either 1 or 0, which for an occupancy matrix translates to either fully occupied or fully unoccupied levels.
Note: in Dudarev's approach the parameters U and J do not enter seperately, only the difference (U-J) is meaningful.
  • LDAUTYPE=4: same as LDAUTYPE=1, but LDA+U instead of LSDA+U (i.e. no LSDA exchange splitting).
In the LDA+U case the double counting energy is given by,

Related Tags and Sections

LDAU, LDAUL, LDAUU, LDAUJ, LDAUPRINT

References


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