The model has a correlation parameter which is estimated and theoretically bound between -1 and +1. To ensure that the estimated parameters are within the theoretical bounds a transformation is necessary. The chosen transformation is:
$$f(\rho): \rho = \frac{2}{(1-exp(-\theta))}- 1$$
Whereas \(\rho\) is the actual correlation coefficient and \(\theta\) is the parameter we estimate in the model. This parametrization has been worked into the likelihood function and ensures that \(\rho\) will be between \(-1\) and \(+1\).