ICHARG

ICHARG = 0 | 1 | 2 | 4 | 5

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=5

External charge-density-update mode to read in and add an external correction to the Kohn-Sham (KS) occupations in every SCF step of the electronic minimization. The initialization of the charge density is done as in ICHARG=1, and after NELMDL steps VASP reads the occupations from a user-supplied text file GAMMA for each k point. The procedure described in Ref.^[1] Eq. (30)-(32) is then used to construct a new charge density from the combined occupations (KS occupations + GAMMA file), from which the next KS potential is constructed. The DFT workflow continues after a user-supplied vasp.lock file is read. Additionally, with ICHARG=5 after each SCF step VASP writes out all with LOCPROJ defined wave function projections. The ICHARG=5 mode can be used with an external code that modifies the occupations, and requires extra output after each SCF step. The TRIQS software package^[2] makes use of it to perform charge self-consistent DFT plus dynamical mean field theory (DMFT) calculations^[3]^[4], and provides tutorials on how to perform such calculations with VASP^[5]^[6].

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.

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