NELECT

NELECT = [real]
Default: NELECT = number of valence electrons 

Description: NELECT sets the number of electrons.

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Warning: Unless you would like to perform a charged calculation, you should not set this line.

The number of electrons is determined automatically from the POTCAR (ZVAL of the element) and POSCAR file (number of the atoms of the respective atom type) assuming the cell is charge-neutral. If the number of electrons is not compatible with the number derived from the valence and the number of atoms a homogeneous background charge is assumed.

Use in the context of charged calculations

Warning: Using the NELECT tag without the use of appropriate correction leads to very slowly convergence of energies with respect to the size L of the super cell.

Change in the total energy as a function of the inverse of the cell dimension (1/L, x-axis) for (left) carbon monoxide computed with a +1 charge (middle) surface of fcc-Al(111) with and without a carbon atom adsorbed on one surface termination (both with +1 charge) and (right) difference of energies between the two middle panels, i.e. difference between energies of fcc-Al(111) with and without carbon computed with a +1 charge.

The required first order correction to the energy caused by an excess charge is given by

$\frac{e^2q^2\alpha}{L\epsilon}$

where q is the net charge of the system, α the Madelung constant of a point charge q placed in a homogeneous background charge -q, and ε the dielectric constant of the system. For atoms or molecules surrounded by vacuum, ε takes on the vacuum value ε=1. VASP can automatically correct for this leading error, see Monopole Dipole and Quadrupole corrections for further information.

It is important to emphasize that the total energy cannot be corrected for charged slabs, since a charged slab results in an electrostatic potential that grows with the distance from the slab. This non-convergence of the potential is a result of of the interaction between the charged slab and the compensating background. A practical consequence of this non-convergence is that the total energy depends linearly on the width of the vacuum. The Figure to the right shows the change in energy as a function of the inverse of the cell dimension for a molecule (carbon monoxide), a surface (fcc-All(111)) with and without the presence of surface adsorbates. In all these cases, the total energy does not converge as the cell dimension is increased.

Warning: Presently, no simple a posteriori correction scheme is known or implemented in VASP for slab calculations. Total energies from charged slab calculations must be used with care. In certain cases relative energies between two charged surface calculations may be useful (see Figure to the right).

Related tags and articles

Monopole Dipole and Quadrupole corrections, EPSILON, DIPOL, IDIPOL, LMONO, LDIPOL, EFIELD

Examples that use this tag

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