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. URL http://www.jstatsoft.org/v43/i09/.

Corresponding BibTeX entry:

  @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},
    url = {http://www.jstatsoft.org/v43/i09/},
  }