To cite optimx in publications use:

John C. Nash, Ravi Varadhan (2011). Unifying Optimization Algorithms to Aid Software System Users: optimx for R. Journal of Statistical Software, 43(9), 1-14. doi 10.18637/jss.v043.i09.

John C. Nash (2014). On Best Practice Optimization Methods in R. Journal of Statistical Software, 60(2), 1-14. doi 10.18637/jss.v060.i02.

Corresponding BibTeX entries:

  @Article{,
    title = {Unifying Optimization Algorithms to Aid Software System
      Users: {optimx} for {R}},
    author = {John C. Nash and Ravi Varadhan},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {43},
    number = {9},
    pages = {1--14},
    doi = {10.18637/jss.v043.i09},
  }

  @Article{,
    title = {On Best Practice Optimization Methods in {R}},
    author = {{John C. Nash}},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {60},
    number = {2},
    pages = {1--14},
    doi = {10.18637/jss.v060.i02},
  }