With the experimental isolation of atomically thin materials, it is now possible measure a variety of charged and neutral multiparticle excitations (trions, biexcitons, etc.) in these systems, many of which display large binding energies. On the theory side, however, while quasiparticle and neutral optical excitations have been successfully treated in the past with the first-principles GW and GW plus Bethe-Salpeter equation (GW-BSE) approaches, respectively, similar atomistic and parameter-free approaches are not available to understand multi-particle excitations. This typically limits theoretical treatments to model Hamiltonians. In this talk, we present results from a new ab initio approach based on the interacting Green’s function formalism to compute multiparticle excitations. Our new diagrammatic approach allows us to predict without adjustable parameters that trions and biexcitons in carbon nanotubes are stable at room temperature, and also reveal in details electronic correlation in these multiparticle excitations. With our new approach, we also show how plasmons in real quasi-two-dimensional metals display a unique but universal dispersion not found in ideal 2D electron gas, which allows for the experimental realization of dispersionless and slowly moving plasmon wave packets. We make some final remarks on how our new theoretical approaches and efficient ab initio implementations can be applied to a wide variety of phenomena and be extended to systems with strong correlations. This work was supported by the Center for Computational Study of Excited-State Phenomena in Energy Materials (C2SEPEM) at LBL, funded by the U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources provided by NERSC and XSEDE.