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).
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.
hess : 2d ndarray
The Hessian matrix, evaluated at params.