  A planetary instability occurring at time $<100$ My after formation of the
giant planets in our solar system can be responsible for some characteristics
of the inner solar system. However, the actual influence of the instability on
the terrestrial planet formation is not well understood. The simulations of
terrestrial planet formation are very CPU-expensive, and this limits the
exploration of different instability scenarios. To include the effects of the
giant planets instability in the simulations of terrestrial planets formation
in a feasible way, we approach the problem in two steps. First, we model and
record an evolution of the giant planets that replicates the present outer
solar system in the end. Then, we use that orbital record, properly
interpolated, as the input for a second step to simulate its effects on the
terrestrial planet formation. For this second step, we developed iSyMBA, a
symplectic massive bodies algorithm, where ``i'' stands for interpolation.
iSyMBA is a very useful code to accurately evaluate the effects of planetary
instabilities on minor body reservoirs, while accounting for close encounters
among massive objects. We provide a detailed description of how iSyMBA was
developed and implemented to study terrestrial planet formation. Adapting
iSyMBA for other problems that demand interpolation from previous simulations
can be done following the method described here.
