To cite package ‘nlopt’ in publications use:

  Gonzalez-Rodriguez B (2023). _nlopt: Call Optimization Solvers with
  .nl Files_. R package version 0.1.1,
  <https://CRAN.R-project.org/package=nlopt>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {nlopt: Call Optimization Solvers with .nl Files},
    author = {Brais Gonzalez-Rodriguez},
    year = {2023},
    note = {R package version 0.1.1},
    url = {https://CRAN.R-project.org/package=nlopt},
  }