  We present ab initio GW plus cumulant-expansion calculations for an organic
compound (TMTSF)2PF6 and a transition-metal oxide SrVO3. These materials
exhibit characteristic low-energy band structures around the Fermi level, which
bring about interesting low-energy properties; the low-energy bands near the
Fermi level are isolated from the other bands and, in the isolated bands,
unusually low-energy plasmon excitations occur. To study the effect of this
low-energy-plasmon fluctuation on the electronic structure, we calculate
spectral functions and photoemission spectra using the ab initio cumulant
expansion of the Green's function based on the GW self-energy. We found that
the low-energy plasmon fluctuation leads to an appreciable renormalization of
the low-energy bands and a transfer of the spectral weight into the incoherent
part, thus resulting in an agreement with experimental photoemission data.
