  We introduce a unified approach to nuclear bound and continuum states based
on the coupling of the no-core shell model (NCSM), a bound-state technique,
with the no-core shell model/resonating group method (NCSM/RGM), a nuclear
scattering technique. This new ab initio method, no-core shell model with
continuum (NCSMC), leads to convergence properties superior to either NCSM or
NCSM/RGM while providing a balanced approach to different classes of states. In
the NCSMC, the ansatz for the many-nucleon wave function includes: i) a
square-integrable A-nucleon component expanded in a complete harmonic
oscillator basis; ii) a binary-cluster component with asymptotic boundary
conditions that can properly describe weakly-bound states, resonances and
scattering; and, in principle, iii) a three-cluster component suitable for the
description of, e.g., Borromean halo nuclei and reactions with final three-body
states. The Schroedinger equation is transformed into a system of
coupled-channel integral-differential equations that we solve using a modified
microscopic R-matrix formalism within a Lagrange mesh basis. We demonstrate the
usefulness of the approach by investigating the unbound 7He nucleus.
