LDAUTYPE: Difference between revisions

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

{\displaystyle E_{\rm HF}=\frac{1}{2} \sum_{\{\gamma\}} (U_{\gamma_1\gamma_3\gamma_2\gamma_4} - U_{\gamma_1\gamma_3\gamma_4\gamma_2}){ \hat n}_{\gamma_1\gamma_2}{\hat n}_{\gamma_3\gamma_4} }

and is determined by the PAW on-site occupancies

{\displaystyle {\hat n}_{\gamma_1\gamma_2} = \langle \Psi^{s_2} \mid m_2 \rangle \langle m_1 \mid \Psi^{s_1} \rangle }

and the (unscreened) on-site electron-electron interaction

{\displaystyle U_{\gamma_1\gamma_3\gamma_2\gamma_4}= \langle m_1 m_3 \mid \frac{1}{|\mathbf{r}-\mathbf{r}^\prime|} \mid m_2 m_4 \rangle \delta_{s_1 s_2} \delta_{s_3 s_4} }

where |m⟩ are real spherical harmonics of angular momentum L=LDAUL.

The unscreened e-e interaction U_γ₁_γ₃_γ₂_γ₄ can be written in terms of the Slater integrals

F^0

,

F^2

,

F^4

, and

F^6

(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

F^0

).

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):

$L\;$	$F^0\;$	$F^2\;$	$F^4\;$	$F^6\;$
$1\;$	$U\;$	$5J\;$	-	-
$2\;$	$U\;$	$\frac{14}{1+0.625}J$	$0.625 F^2\;$	-
$3\;$	$U\;$	$\frac{6435}{286+195 \cdot 0.668+250 \cdot 0.494}J$	$0.668 F^2\;$	$0.494 F^2\;$

The essence of the LSDA+U method consists of the assumption that one may now write the total energy as:

{\displaystyle E_{\mathrm{tot}}(n,\hat n)=E_{\mathrm{DFT}}(n)+E_{\mathrm{HF}}(\hat n)-E_{\mathrm{dc}}(\hat n) }

where the Hartree-Fock like interaction replaces the LSDA on site due to the fact that one subtracts a double counting energy

E_{\mathrm{dc}}

, which supposedly equals the on-site LSDA contribution to the total energy,

{\displaystyle E_{\mathrm{dc}}(\hat n) = \frac{U}{2} {\hat n}_{\mathrm{tot}}({\hat n}_{\mathrm{tot}}-1) - \frac{J}{2} \sum_\sigma {\hat n}^\sigma_{\mathrm{tot}}({\hat n}^\sigma_{\mathrm{tot}}-1). }

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:

{\displaystyle E_{\mathrm{LSDA+U}}=E_{\mathrm{LSDA}}+\frac{(U-J)}{2}\sum_\sigma \left[ \left(\sum_{m_1} n_{m_1,m_1}^{\sigma}\right) - \left(\sum_{m_1,m_2} \hat n_{m_1,m_2}^{\sigma} \hat n_{m_2,m_1}^{\sigma} \right) \right]. }

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,

\hat n^{\sigma} = \hat n^{\sigma} \hat n^{\sigma}

.

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,

{\displaystyle E_{\mathrm{dc}}(\hat n) = \frac{U}{2} {\hat n}_{\mathrm{tot}}({\hat n}_{\mathrm{tot}}-1) - \frac{J}{2} \sum_\sigma {\hat n}^\sigma_{\mathrm{tot}}({\hat n}^\sigma_{\mathrm{tot}}-1). }

Related Tags and Sections

LDAU, LDAUL, LDAUU, LDAUJ, LDAUPRINT

References

Contents

[liechtenstein:prb:95-1] A. I. Liechtenstein, V. I. Anisimov and J. Zaane, Phys. Rev. B 52, R5467 (1995).

[dudarev:prb:98-2] S. L. Dudarev, G. A. Botton, S. Y. Savrasov, C. J. Humphreys and A. P. Sutton, Phys. Rev. B 57, 1505 (1998).

[1]

[2]

@@ Line 3: / Line 3: @@
 Description: {{TAG|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 {{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"/>
 :This particular flavour of LSDA+U is of the form