# KERNEL

Defines how the kernel is built in terms of functionals. [Edit on GitHub]

Subsections

- EXACT_EXCHANGE
- XC_FUNCTIONAL

## Keywords

- RI_REGION

## Keyword descriptions

RI_REGION: real = 0.00000000E+000

  Aliases: RI_RADIUS

  Usage: RI_REGION {real}

  The region defined by a sphere of the given radius around the excited atom defining which RI_XAS basis elements are considered for the RI projection of the density. Each basis element which center is in this region is taken. The density for a given excited atom is expressed as : sum_ijkl P_ij (ijk) S_kl^-1 xi_l, where P_ij is the density matrix, i,j span the orbital basis and k,l the RI_XAS basis in the region. The larger the radius, the more basis functions to expand the density. However, it is assumed that it is a small number and the code does not scale well as the number of RI basis elements gets too large. Expressed in Angstrom. If the radius is set to 0.0, only the RI basis elements centered on the excited atom are used. [Edit on GitHub]