Thanks for the quick response. Indeed, the contribution of this term is very small (two orders of magnitude compared to the Bond or Angle term).
I am just comparing with the help:
http://lammps.sandia.gov/doc/improper_class2.html
there we have: Theta ijl – theta 2, that is equivalent to Tetha ABD – aa_theta0_2[type] but in the code it says Tetha ABD – aa_theta0_3[type] (line 706 – Improper_class2.cpp), so it may be wrong in the help or maybe there is something I don't underestand.
Fernando