To cite the nlmeU package in publications use:

Galecki A, Burzykowski T (2013). Linear Mixed-Effects Models Using R: A Step-by-Step Approach, First edition. Springer, New York. ISBN 978-1-4614-3899-1, https://link.springer.com/book/10.1007/978-1-4614-3900-4.

Corresponding BibTeX entry:

  @Book{,
    title = {Linear Mixed-Effects Models Using R: A Step-by-Step
      Approach},
    author = {Andrzej Galecki and Tomasz Burzykowski},
    publisher = {Springer},
    edition = {First},
    address = {New York},
    year = {2013},
    note = {ISBN 978-1-4614-3899-1},
    url = {https://link.springer.com/book/10.1007/978-1-4614-3900-4},
  }