If you use results obtained with EasySpin in any scientific publication, cite

Stefan Stoll, Arthur Schweiger
EasySpin, a comprehensive software package for spectral simulation and analysis in EPR
J. Magn. Reson. 178(1), 42-55 (2006)
[pdf file]

EasySpin's pulse simulation capabilities are described in detail in

Stefan Stoll, R. David Britt
General and efficient simulation of pulse EPR spectra
Phys. Chem. Chem. Phys. 11, 6614-6625 (2009)
[pdf file]

More material about EasySpin:

EPR Simulations with EasySpin
Lecture slides, CAESR workshop, Oxford, England, September 2009
[pdf file]

EasySpin: Pulse EPR Simulations
Lecture slides, COST workshop on pulse simulations, Norwich, England, April 2009
[pdf file]

EasySpin: Software for Electron Paramagnetic Resonance
Lecture slides, 4th International EPR Summer School, St. Andrews, Scotland, August 2008
[pdf file]

Stefan Stoll, Arthur Schweiger
EasySpin: Simulating cw ESR spectra
Biol. Magn. Reson. 27, 299-321 (2007)
[pdf file]

Other articles about about spectral simulation in EPR and the algorithms used in EasySpin are cited in the above articles.