The very forward calorimeter of the CMS experiment CASTOR measures the largest energy densities accessible at the LHC and is a unique tool to study soft multiparticle production relevant for the underlying event and air shower development. Within this work, the very forward collision energy is measured in proton-proton collisions at 13 TeV. The results are given in terms of the differential production cross section and as a function of the charged particle multiplicity at central rapidity. The data are furthermore used to derive implications on hadronic interaction models
