To cite optimParallel in publications use:

  F. Gerber and R. Furrer (2019). optimParallel: An R package providing
  a parallel version of the L-BFGS-B optimization method. The R
  Journal, 11(1):352-358, 2019, https://doi.org/10.32614/RJ-2019-030

A BibTeX entry for LaTeX users is

  @Article{,
    title = {optimParallel: An R Package Providing a Parallel Version of the L-BFGS-B Optimization Method},
    author = {Florian Gerber and Reinhard Furrer},
    journal = {The R Journal},
    year = {2019},
    volume = {11},
    number = {1},
    pages = {352--358},
    url = {https://doi.org/10.32614/RJ-2019-030},
    doi = {10.32614/RJ-2019-030},
  }
