.. _model-surrogate-global-gaussian_process-surfpack-nugget:

""""""
nugget
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Specify a nugget to handle ill-conditioning


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**Specification**

- *Alias:* None

- *Arguments:* 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 :dakkw:`model-surrogate-global-gaussian_process-surfpack-nugget`.


- Have Surfpack find the optimal value of the nugget. This is specified by :dakkw:`model-surrogate-global-gaussian_process-surfpack-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.