To cite the nnet package in publications use:

Ripley BD, Venables WN (2002). Modern Applied Statistics with S, Fourth edition. Springer, New York. ISBN 0-387-95457-0, https://www.stats.ox.ac.uk/pub/MASS4/.

Corresponding BibTeX entry:

  @Book{,
    entry = {Book},
    title = {Modern Applied Statistics with S},
    author = {Brian D. Ripley and William N. Venables},
    publisher = {Springer},
    edition = {Fourth},
    address = {New York},
    year = {2002},
    note = {ISBN 0-387-95457-0},
    url = {https://www.stats.ox.ac.uk/pub/MASS4/},
  }