Rainer Brüggemann, a pioneer in mathematical order theory to address chemical and environmental questions, passed away on December 4, 2022, at the age of 79 after being diagnosed with cancer in May 2022. He had a scientific inclination from a young age and studied chemistry at the Ludwig Maximilian University of Munich, where he obtained his doctoral degree in chemistry in 1977. He was an early advocate of the powers of computation in chemistry and environmental sciences and was committed to open science by sharing his software package PyHasse. Rainer entered the field of environmental sciences as a researcher working on the disposal of nuclear waste in salt domes at the Technical University of Berlin. He later moved to the Gesellschaft für Strahlen- und Umweltforschung, where he began an influential collaboration with Efraim Halfon, which produced several publications, collaborations, and PhD works. He moved back to Berlin in the 1990s as a senior researcher at the Leibniz Institute of Freshwater Ecology and Inland Fisheries and lectured at Bayreuth University. Rainer remained active until his retirement in the 2000s and engaged in his last official work as a remote teacher at Sudile. He was known for his energy, kindness, sense of humor, and sincere efforts to positively contribute to the lives of his family, colleagues, and students.