The present work reports the theoretical background and the capabilities of SEISMIC, a Python code specifically developed to calculate the propagation of the sound waves inside crystalline materials. SEISMIC is a tool integrated in the Quantas package and provides a series of useful information for engineers and geophysicists, such as the phase and group velocities, the power flow angle, the enhancement factors, and the polarization vectors, using as input the elastic moduli and the density of the material. Numerical treatments of the derivatives were avoided, using analytical methods to obtain numerically stable results. The code relies only on Python numerical and graphical libraries to ensure a full cross-platform usability. •Automated software to calculate the propagation of seismic waves in crystalline solids.•Analytical evaluation of the second derivatives, which improve the description.•Entirely coded in Python, with no need of external software for plotting.•Integrated in Quantas, a software suite for the analysis of solid phases.•The code is completely cross-platform.