To cite optimx in publications use:

Nash JC, Varadhan R (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.

Nash JC (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},
  }