The era of gravitational wave astronomy began a few years ago with the first direct detections by the LIGO/Virgo collaboration. These detections and the accompanying understanding result from detailed knowledge of expected gravitational waveforms, and numerical evolutions are a critical ingredient in this regard. This project studies a primary source of gravitational waves for the recently upgraded LIGO detectors, namely the merger and inspiral of neutron stars. The merger of binary neutron stars is also responsible for concurrent electromagnetic signals to which conventional telescopes are sensitive, so called multi-messenger astronomy. Simulations will be carried out to study both expectations for gravitational and electromagnetic signals.<br/><br/>Various aspects of such mergers will be studied in detail. For example, phase transitions within the neutron stars may affect expected signals during the merger. Using a piecewise-polytrope to model the equation of state with and without a phase transition, evolutions will reveal possible differences. Other effects to be studied include multiscale approximations of magnetic instabilities and axion clouds. These studies will prove helpful not only to the gravitational wave data analysis community but to the astronomical community as well. The activities described here will help further undergraduate, graduate, and postdoctoral research and STEM training, including a number of underrepresented individuals (continuing an existing record of such mentoring). Also, this work builds on open-source infrastructure and other open-source tools, and thus it helps support broadly useful, publicly released, software.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.