To cite NbClust in publications use:

Charrad M, Ghazzali N, Boiteau V, Niknafs A (2014). “NbClust: An R Package for Determining the Relevant Number of Clusters in a Data Set.” Journal of Statistical Software, 61(6), 1–36. https://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 = {https://www.jstatsoft.org/v61/i06/},
  }