  Microlensing of stars in our Galaxy has long been used to detect and
characterize stellar populations, exoplanets, brown dwarfs, stellar remnants
and whatever objects may magnify the source stars with their gravitational
fields. The interpretation of microlensing light curves is relatively simple
for single lenses and single sources but becomes more and more complicated if
we add more objects and take their relative motion into account. RTModel is a
modeling platform that has been very active in the real-time investigation of
microlensing events, providing preliminary models that have proven very useful
for driving follow-up resources towards the most interesting events. The
success of RTModel is due to the ability to make a thorough and aimed
exploration of the parameter space in a relatively short time. This is obtained
by three key ideas: the initial conditions are chosen from a template library
including all possible caustic crossing and approaches; the fits are performed
by the Levenberg-Marquardt algorithm using a bumper mechanism to explore
multiple minima; the basic computations of microlensing magnification are
performed by the fast and robust VBBinaryLensing package. In this paper we will
illustrate all algorithms in RTModel in detail, with the purpose of fostering
new ideas in view of future microlensing pipelines aimed at massive
microlensing analysis.
