Multiscale computer simulations combine the computationally efficient classical algorithms with more expensive--and more accurate--ab initio quantum mechanical algorithms. Here, the authors describe one implementation of multiscale computations using the Atomistic Simulation Environment (ASE). This implementation can mix classical codes including the large-scale atomic/molecular massively parallel simulator (LAMMPS) and the density functional theory-based projector-augmented wave (PAW) implementation designed to work with ASE. Any combination of codes linked via the ASE interface can be mixed. The authors introduce a framework to easily add classical force-fields calculators for ASE using LAMMPS, which also allows harnessing the full performance of classical-only molecular dynamics. Their work makes it possible to combine different simulation codes (quantum mechanical or classical) with great ease and minimal coding effort.