To cite optimParallel in publications use:

F. Gerber and R. Furrer (2018) optimParallel: an R Package Providing Parallel Versions of the Gradient-Based Optimization Methods of optim(). URL http://arxiv.org/abs/1804.11058

Corresponding BibTeX entry:

  @Article{,
    title = {optimParallel: an R Package Providing Parallel Versions of
      the Gradient-Based Optimization Methods of optim()},
    author = {Florian Gerber and Reinhard Furrer},
    journal = {ArXiv e-prints},
    year = {2018},
    pages = {1--4},
    url = {http://arxiv.org/abs/1804.11058},
  }