An approach is suggested for the investigation by computer simulation of systems which contain free electrons. Summation over the occupied free electron states makes it possible to exclude quantum particles from explicit consideration. This procedure leads to renormalized effective interatomic potentials which are generally weaker than the initial direct ones and allows us to apply computer simulation methods developed for classical systems to the study of systems with free electrons. The approach suggested is illustrated by molecular dynamics studies of concentrated lithium-ammonia solutions.