# RI_METRIC

This subsection allows for the definition of an exchange RI metric that is different from the main exchange potential. By default (i.e. if this subsection is ignored), the exchange kernel is computed in the V approximation: (ab|ij) = (ab|P) V^-1 (Q|ij), where V = (P|Q). With a RI metric, we have a 2 step RI involving the metric potential for the 3-center integrals: (ab|ij) = (ab!P) (P!Q)^-1 (Q|R) (R!S)^-1 (S!ij), where | stands for the exchange potential and ! for the metric potential. This allows for drastic screening of the 3-center integrals by selecting shorter range metric. [Edit on GitHub]

## Keywords

- SECTION_PARAMETERS

- CUTOFF_RADIUS

- OMEGA

- POTENTIAL_TYPE

- T_C_G_DATA

## Keyword descriptions

SECTION_PARAMETERS: logical = F

  Lone keyword: `T`

  Usage: &EXACT_EXCHANGE {logical}

  Enables the use of a RI metric. [Edit on GitHub]

CUTOFF_RADIUS: real = 0.00000000E+000

  Aliases: R_C ,RC ,RANGE

  Usage: CUTOFF_RADIUS {double}

  The cutoff radius (in Angstrom) for the truncated Coulomb operator. [Edit on GitHub]

OMEGA: real = 0.00000000E+000

  Usage: OMEGA {double}

  The range parameter for the short range operator (in 1/a0). [Edit on GitHub]

POTENTIAL_TYPE: enum = OVERLAP

  Aliases: OP ,OPERATOR ,POTENTIAL

  Usage: OPERATOR {string}

  Valid values:

  - `OVERLAP` Overlap operator (=identity)

  - `TRUNCATED` Truncated Coulomb operator: 1/r if r < R_c, 0 otherwise

  - `SHORTRANGE` Short range: erfc(omega*r)/r

  The type of operator used for the metric. [Edit on GitHub]

T_C_G_DATA: string = t_c_g.dat

  Usage: T_C_G_DATA {string}

  Location of the file t_c_g.dat that contains the data for the evaluation of the truncated gamma function [Edit on GitHub]