merDeriv is a free, open source R package for computing derivatives of mixed models estimated via lme4. Allows users to compute scores (casewise first derivatives of likelihood function) and variance-covariance matrix of all estimated parameters, including random effect (co)variances.

For further information, see:

Wang, T., & Merkle, E. C. (in press). Derivative computations and robust standard errors for linear mixed effects models in lme4. Journal of Statistical Software.