To cite systemfit in publications use:

Arne Henningsen and Jeff D. Hamann (2007). systemfit: A Package for Estimating Systems of Simultaneous Equations in R. Journal of Statistical Software 23(4), 1-40. URL http://www.jstatsoft.org/v23/i04/.

Corresponding BibTeX entry:

  @Article{,
    title = {systemfit: A Package for Estimating Systems of
      Simultaneous Equations in R},
    author = {Arne Henningsen and Jeff D. Hamann},
    journal = {Journal of Statistical Software},
    year = {2007},
    volume = {23},
    number = {4},
    pages = {1--40},
    url = {http://www.jstatsoft.org/v23/i04/},
  }