Yes, I am using MAtlab but I am using two algorithms founded on the internet. If you do the MSUP with basi excitation (imposing the impulse acceleration), you can verify that the final displacement is the same of the double integration. In Matlab I am using two different algorithms in order to obtain the spectrum of the impulse data (I am working with a Shock impulse with half-sine shape - 10ms_200G). The two algorithms are based on two methods: Kelly-Richman and Smallwood. nSorry but I did not understand the answer to my doubt:nTo apply the acceleration to the entire model (acceleration - all bodies) fixing the base nodes is the same of to apply the acceleration to only the fixed points (screws points ) ?n