Dakota Reference Manual: nugget

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nugget

Specify a nugget to handle ill-conditioning

## Specification

Alias: none

Argument(s): REAL

Default: None

## Description

By default, the Surfpack GP handles ill-conditioning and does not use a nugget. If the user wishes to specify a nugget, there are two approaches.

- The user can specify the value of a nugget with nugget.
- Have Surfpack find the optimal value of the nugget. This is specified by find_nugget. There are two options for find_nugget.
  - `find_nugget` = 1: assume that the reciprocal condition number of the correlation matrix R, rcondR, is zero and calculate the nugget needed to make the worst case of R not ill-conditioned.
  - `find_nugget` = 2: calculate rcondR, which requires a Cholesky factorization. If rcondR indicates that R is not ill-conditioned, then kriging uses the Cholesky factorization. Otherwise, if rcondR says R is ill conditioned, then kriging will calculate the nugget needed to make the worst case of R not ill conditioned.
  `find_nugget` = 1 and 2 are similar, the second option just takes more computation (the initial Cholesky factorization) for larger problems.