To cite HLMdiag in publications use:

Loy A, Hofmann H (2014). “HLMdiag: A Suite of Diagnostics for Hierarchical Linear Models in R.” Journal of Statistical Software, 56(5), 1–28. https://www.jstatsoft.org/article/view/v056i05.

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 = {https://www.jstatsoft.org/article/view/v056i05},
  }