ICHARG: Difference between revisions

Latest revision as of 15:17, 30 December 2023

ICHARG = 0 | 1 | 2 | 4

Default: ICHARG	= 2	if ISTART=0
	= 0	else

Description: ICHARG determines how VASP constructs the initial charge density.

ICHARG=0

Calculate the charge density from initial wave functions.

If ISTART is internally reset due to an invalid WAVECAR file, ICHARG will be set to ICHARG=2.

ICHARG=1

Read the charge density from CHGCAR file, and extrapolate from the old positions (on CHGCAR) to the new positions using a linear combination of atomic charge densities.

In the PAW method, there is, however, one important point to keep in mind: For the on-site densities (that is, the densities within the PAW sphere), only l-decomposed charge densities up to LMAXMIX are written. Upon restart, the energies might, therefore, differ slightly from the fully converged energies. The discrepancies can be large for the DFT+U method. In this case, one might need to increase LMAXMIX to 4 (d-elements) or even 6 (f-elements).

Tip: To improve convergence and reduce the number of electronic steps, it is recommended to set ICHARG = 1 when starting calculations repeatedly with small changes in the input parameters.

ICHARG=2

Take superposition of atomic charge densities.

ICHARG=4

Read potential from file POT. The local potential on the file POT is written by the optimized-effective-potential methods (OEP), if the flag LVTOT=.TRUE. is supplied in the INCAR file. Supported as of VASP.5.1.

ICHARG+10

non-selfconsistent calculations: Adding 10 to the value of ICHARG, e.g., ICHARG=11 or 12 (or the less convenient value 10) means that the charge density will be kept constant during the entire electronic minimization.

There are several reasons why to keep the charge density constant:

ICHARG=11

To obtain the eigenvalues (for band-structure plots) or the density of states (DOS) of a given charge density read from CHGCAR. The self-consistent CHGCAR file must be determined beforehand by a fully self-consistent calculation with a k-point grid spanning the entire Brillouin zone.

ICHARG=12

Non-self-consistent calculations for a superposition of atomic charge densities. This is in the spirit of the non-self-consistent Harris-Foulkes functional. The stress and the forces calculated by VASP are correct, and it is possible to perform an ab-initio MD for the non-selfconsistent Harris-Foulkes functional.

Tip: If ICHARG is set to 11 or 12, it is strongly recommended to set LMAXMIX to twice the maximum l-quantum number in the pseudopotentials. Thus, for s and p elements LMAXMIX should be set to 2, for d elements LMAXMIX should be set to 4, and for f elements LMAXMIX should be set to 6.

The initial charge density is of importance in the following cases:

If ICHARG≥10 the charge density remains constant during the run.

For all algorithms except IALGO=5X the initial charge density is used to set up the initial Hamiltonian that is used in the first few non-selfconsistent steps, c.f., NELMDL tag.

@@ Line 5: / Line 5: @@
 ----
 *{{TAG|ICHARG}}=0
-:Calculate charge density from initial wave functions.
+:Calculate the charge density from initial wave functions.
-:If {{TAG|ISTART}} is ''internally reset'' due to an invalid {{FILE|WAVECAR}}-file {{TAG|ICHARG}} will be set to {{TAG|ICHARG}}=2.
+:If {{TAG|ISTART}} is ''internally reset'' due to an invalid {{FILE|WAVECAR}} file, {{TAG|ICHARG}} will be set to {{TAG|ICHARG}}=2.
 *{{TAG|ICHARG}}=1
-:Read the charge density from file {{FILE|CHGCAR}}, and extrapolate from the old positions (on {{FILE|CHGCAR}}) to the new positions using a linear combination of atomic charge densities.
+:Read the charge density from {{FILE|CHGCAR}} file, and extrapolate from the old positions (on {{FILE|CHGCAR}}) to the new positions using a linear combination of atomic charge densities.
-:In the PAW method, there is however one important point to keep in mind. For the on-site densities (that is the densities within the PAW sphere) only l-decomposed charge densities up to {{TAG|LMAXMIX}} are written. Upon restart the energies might therefore differ slightly from the fully converged energies. The discrepancies can be large for the L(S)DA+U method. In this case, one might need to increase {{TAG|LMAXMIX}} to 4 (d-elements) or even 6 (f-elements).
+:In the [[Projector-augmented-wave_formalism|PAW method]], there is, however, one important point to keep in mind: For the on-site densities (that is, the densities within the PAW sphere), only l-decomposed charge densities up to {{TAG|LMAXMIX}} are written. Upon restart, the energies might, therefore, differ slightly from the fully converged energies. The discrepancies can be large for the DFT+U method. In this case, one might need to increase {{TAG|LMAXMIX}} to 4 (d-elements) or even 6 (f-elements).
+{{NB|tip|To improve convergence and reduce the number of electronic steps, it is recommended to set ICHARG {{=}} 1 when starting calculations repeatedly with small changes in the input parameters.|:}}
 *{{TAG|ICHARG}}=2
-:Take superposition of atomic charge densities
+:Take superposition of atomic charge densities.
 *{{TAG|ICHARG}}=4
-:Supported as of VASP.5.1: read potential from file {{FILE|POT}}. The local potential on the file {{FILE|POT}} is written by the optimized effective potential methods (OEP), if the flag {{TAG|LVTOT}}=.TRUE. is supplied in the {{FILE|INCAR}} file.
+:Read potential from file {{FILE|POT}}. The local potential on the file {{FILE|POT}} is written by the optimized-effective-potential methods (OEP), if the flag {{TAG|LVTOT}}=.TRUE. is supplied in the {{FILE|INCAR}} file. Supported as of VASP.5.1.
 *{{TAG|ICHARG}}+10
-:non-selfconsistent calculations: Adding 10 to the value of {{TAG|ICHARG}}, e.g. {{TAG|ICHARG}}=11 or 12 (or the less convenient value 10) means that the charge density will be kept constant during the ''whole electronic minimization''.
+:non-selfconsistent calculations: Adding 10 to the value of {{TAG|ICHARG}}, e.g., {{TAG|ICHARG}}=11 or 12 (or the less convenient value 10) means that the charge density will be kept constant during the ''entire electronic minimization''.
-:There are several reasons why to use this flag:
+:There are several reasons why to keep the charge density constant:
 :*{{TAG|ICHARG}}=11
-::To obtain the eigenvalues (for band structure plots) or the DOS for a given charge density read from {{FILE|CHGCAR}}. The selfconsistent {{FILE|CHGCAR}} file must be determined beforehand doing by a fully self-consistent calculation  with a k-point grid spanning the entire Brillouin zone.\ref{ex-band}.
+::To obtain the eigenvalues (for band-structure plots) or the density of states (DOS) of a given charge density read from {{FILE|CHGCAR}}. The self-consistent {{FILE|CHGCAR}} file must be determined beforehand by a fully self-consistent calculation with a k-point grid spanning the entire Brillouin zone.
 :*{{TAG|ICHARG}}=12
-::Non-selfconsistent calculations for a superposition of atomic charge densities. This is in the spirit of the  non-selfconsistent Harris-Foulkes functional. The stress and the forces calculated by VASP are correct, and it is absolutely possible to perform an ab-initio MD for the non-selfconsistent [[Harris-Foulkes functional]].
+::Non-self-consistent calculations for a superposition of atomic charge densities. This is in the spirit of the non-self-consistent [[Harris-Foulkes functional|Harris-Foulkes functional]]. The stress and the forces calculated by VASP are correct, and it is possible to perform an ab-initio MD for the non-selfconsistent [[Harris-Foulkes functional|Harris-Foulkes functional]].
+{{NB|tip|If {{TAG|ICHARG}} is set to 11 or 12, it is strongly recommended to set {{TAG|LMAXMIX}} to twice the maximum l-quantum number in the pseudopotentials. Thus, for s and p elements {{TAG|LMAXMIX}} should be set to 2, for d elements {{TAG|LMAXMIX}} should be set to 4, and for f elements {{TAG|LMAXMIX}} should be set to 6.|:}}
+The initial charge density is of importance in the following cases:
-:If {{TAG|ICHARG}} is set to 11 or 12, it is strongly recommended to set {{TAG|LMAXMIX}} to twice the maximum l-quantum number in the pseudpotentials. Thus for s and p elements {{TAG|LMAXMIX}} should be set to 2, for d elements {{TAG|LMAXMIX}} should be set to 2, and for f elements {{TAG|LMAXMIX}} should be set to 6.
+*If {{TAG|ICHARG}}&ge;10 the charge density remains constant during the run.
+*For all algorithms except {{TAG|IALGO}}=5X the initial charge density is used to set up the initial Hamiltonian that is used in the first few non-selfconsistent steps, c.f., {{TAG|NELMDL}} tag.
-The initial charge density is of importance in the following cases:
+== Related tags and articles ==
-*If {{TAG|ICHARG}}&ge;10 the charge density remains constant during the run.
+{{FILE|CHGCAR}}, {{TAG|ISTART}}, {{TAG|LCHARG}}, {{TAG|LMAXMIX}}, {{TAG|NELMDL}}, {{TAG|INIWAV}}
-*For all algorithms except {{TAG|IALGO}}=5X the initial charge density is used to set up the initial Hamiltonian which is used in the first few ({{TAG|NELMDL}}) non selfconsistent steps.
+{{sc|ICHARG|Examples|Examples that use this tag}}
 ----
-[[The_VASP_Manual|Contents]]
-[[Category:INCAR]]
+[[Category:INCAR tag]][[Category:Electronic minimization]][[Category:Charge density]]