To cite NbClust in publications use:

Malika Charrad, Nadia Ghazzali, Veronique Boiteau, Azam Niknafs (2014). NbClust: An R Package for Determining the Relevant Number of Clusters in a Data Set. Journal of Statistical Software, 61(6), 1-36. URL http://www.jstatsoft.org/v61/i06/.

Corresponding BibTeX entry:

  @Article{,
    title = {{NbClust}: An {R} Package for Determining the Relevant
      Number of Clusters in a Data Set},
    author = {Malika Charrad and Nadia Ghazzali and V{\'e}ronique
      Boiteau and Azam Niknafs},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {61},
    number = {6},
    pages = {1--36},
    url = {http://www.jstatsoft.org/v61/i06/},
  }