  No-Core Gamow Shell Model (NCGSM) is applied for the first time to study
selected well-bound and unbound states of helium isotopes. This model is
formulated on the complex energy plane and, by using a complete Berggren
ensemble, treats bound, resonant, and scattering states on equal footing. We
use the Density Matrix Renormalization Group method to solve the many-body
Schr\"{o}dinger equation. To test the validity of our approach, we benchmarked
the NCGSM results against Faddeev and Faddeev-Yakubovsky exact calculations for
$^3$H and $^4$He nuclei. We also performed {\textit ab initio} NCGSM
calculations for the unstable nucleus $^5$He and determined the ground state
energy and decay width, starting from a realistic N$^3$LO chiral interaction.
