To cite HLMdiag in publications use:

Adam Loy, Heike Hofmann (2014). HLMdiag: A Suite of Diagnostics for Hierarchical Linear Models in R. Journal of Statistical Software, 56(5), 1-28. URL http://www.jstatsoft.org/v56/i05/.

Corresponding BibTeX entry:

  @Article{,
    title = {{HLMdiag}: A Suite of Diagnostics for Hierarchical Linear
      Models in {R}},
    author = {Adam Loy and Heike Hofmann},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {56},
    number = {5},
    pages = {1--28},
    url = {http://www.jstatsoft.org/v56/i05/},
  }