Returns the model’s Hessian matrix.

Calculates the Hessian matrix for the linear mixed effects model with respect to the parameterization in which the covariance matrix is represented directly (without square-root transformation).

Parameters:params (MixedLMParams or array-like) – The model parameters at which the Hessian is calculated. If array-like, must contain the packed parameters in a form that is compatible with this model instance.
Returns:hess – The Hessian matrix, evaluated at params.
Return type:2d ndarray