Owing to the fact that a Living Reviews in Solar Physics article may evolve over time, we recommend to cite the article as follows:
Thomas Wiegelmann and Takashi Sakurai,
"Solar Force-free Magnetic Fields",
Living Rev. Solar Phys. 9, (2012), 5. URL (accessed <date>):
The date given as <date> then uniquely identifies the version of the article you are referring to.