We were deeply shocked and saddened to hear that our colleague, friend and mentor Prof. Walter Thiel passed away last month. Walter Thiel made important contributions to many areas of computational chemistry over his long career, but ChemShell users will probably know him best from his crucial role in the development and application of combined quantum mechanical/molecular mechanical (QM/MM) methods to problems in biomolecular modelling. In addition to developing many of the underlying models which accurately describe such systems, he was a key developer of the ChemShell QM/MM package, beginning in 1998 with the EU-funded QUASI collaboration which introduced a range of new features into the program targeting enzymatic reactions. His semi-empirical electronic structure package MNDO was integrated with ChemShell and became a highly successful workhorse for enzyme active sites in the context of QM/MM. He subsequently pushed forward QM/MM methodology into areas such as molecular dynamics calculations, free-energy methods, microiterative optimisation techniques, solvent boundary potentials and methods for excited state optimisation and dynamics. His group applied these methods to a wide range of biomolecules, including of particular note a long series of investigations into the cytochrome P450 family of enzymes. Many other research groups followed his lead, resulting in hundreds of research publications which would not have existed without his pioneering work.

Walter Thiel continued to make major new contributions right up until his recent retirement from the MPI für Kohlenforschung in Mülheim, with recent method developments involving fully polarisable QM/MM models of enzymes, triple resolution QM/MM/coarse grained simulations, and QM/MM with periodic boundary conditions. His influence will continue to be felt for many years to come through the postgraduate students and postdoctoral researchers he mentored in Mülheim who have gone on to establish their own research programmes. We will also remember the special bond between the MPI and STFC Daresbury Laboratory, with several past and current Daresbury researchers having previously worked in his group. He was a wonderfully supportive, kind and inspiring supervisor and collaborator and will be greatly missed by us all.