  We propose a new unfolding scheme to analyze energy spectra of complex
large-scale systems which are inherently of multi-periodicity. Considering
twisted bilayer graphene (tBLG) as an example, we first show that the
conventional unfolding scheme in the past using a single primitive-cell
representation causes serious problems in analyses of the energy spectra. We
then introduce our multi-space representation scheme in the unfolding method
and clarify its validity for tBLG. Velocity renormalization of Dirac electrons
in tBLG is elucidated in the present unfolding scheme.
