To cite optimParallel in publications use:

Gerber F, Furrer R (2019). “optimParallel: An R Package Providing a Parallel Version of the L-BFGS-B Optimization Method.” The R Journal, 11(1), 352–358. doi: 10.32614/RJ-2019-030, https://doi.org/10.32614/RJ-2019-030.

Corresponding BibTeX entry:

  @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},
  }