I will present how ab initio electronic structure and multichannel scattering calculations can propose, guide, and explain ultracold quantum physics experiments. I will start with our theoretical predictions of the scattering lengths for ultracold collisions without any adjustment to experimental data, which were possible by including high-level-correlation, relativistic, QED, and adiabatic corrections. I will conclude with our theoretical explanations of quantum resonances measured in ultracold atom-ion and atom-molecule collisions, where the quantum regime of cold collisions and their quantum control with an external magnetic field have been recently reached for the first time.