To cite IsoSpecR in publications use:

  Mateusz K. Łącki, Dirk Valkenborg, Michał Startek (2020). “IsoSpec2:
  Ultrafast Fine Structure Calculator.” _Analytical Chemistry_,
  *92*(14), 9472-9475. doi:10.1021/acs.analchem.0c00959
  <https://doi.org/10.1021/acs.analchem.0c00959>. Mateusz K. Łącki,
  Dirk Valkenborg, Michał Startek (2020). IsoSpec2: Ultrafast Fine
  Structure Calculator Analytical Chemistry, Published online ahead of
  print URL https://doi.org/10.1021/acs.analchem.0c00959,
  <https://doi.org/10.1021/acs.analchem.0c00959>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{IsoSpec2}: Ultrafast Fine Structure Calculator},
    author = {{Mateusz K. Łącki} and {Dirk Valkenborg} and {Michał Startek}},
    year = {2020},
    journal = {Analytical Chemistry},
    volume = {92},
    number = {14},
    pages = {9472-9475},
    doi = {10.1021/acs.analchem.0c00959},
    url = {https://doi.org/10.1021/acs.analchem.0c00959},
    note = {Mateusz K. Łącki, Dirk Valkenborg, Michał Startek (2020). IsoSpec2: Ultrafast Fine Structure Calculator Analytical Chemistry, Published online ahead of print URL https://doi.org/10.1021/acs.analchem.0c00959},
  }

  Mateusz K. Łącki, Michał Startek, Dirk Valkenborg, Anna Gambin
  (2017). “IsoSpec: Hyperfast Fine Structure Calculator.” _Analytical
  Chemistry_, *89*(6), 3272-3277. Mateusz K. Łącki, Michał Startek,
  Dirk Valkenborg, Anna Gambin (2017). IsoSpec: Hyperfast Fine
  Structure Calculator. Analytical Chemistry 89(6), 3272-3277. URL
  https://doi.org/10.1021/acs.analchem.6b01459,
  <https://doi.org/10.1021/acs.analchem.6b01459>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{IsoSpec}: Hyperfast Fine Structure Calculator},
    author = {{Mateusz K. Łącki} and {Michał Startek} and {Dirk Valkenborg} and {Anna Gambin}},
    journal = {Analytical Chemistry},
    year = {2017},
    volume = {89},
    number = {6},
    pages = {3272-3277},
    url = {https://doi.org/10.1021/acs.analchem.6b01459},
    note = {Mateusz K. Łącki, Michał Startek, Dirk Valkenborg, Anna Gambin (2017). IsoSpec: Hyperfast Fine Structure Calculator. Analytical Chemistry 89(6), 3272-3277. URL https://doi.org/10.1021/acs.analchem.6b01459},
  }