  \textsc{Quantum Package} is an open-source programming environment for
quantum chemistry specially designed for wave function methods. Its main goal
is the development of determinant-driven selected configuration interaction
(sCI) methods and multi-reference second-order perturbation theory (PT2). The
determinant-driven framework allows the programmer to include any arbitrary set
of determinants in the reference space, hence providing greater methodological
freedoms. The sCI method implemented in \textsc{Quantum Package} is based on
the CIPSI (Configuration Interaction using a Perturbative Selection made
Iteratively) algorithm which complements the variational sCI energy with a PT2
correction. Additional external plugins have been recently added to perform
calculations with multireference coupled cluster theory and range-separated
density-functional theory. All the programs are developed with the IRPF90 code
generator, which simplifies collaborative work and the development of new
features. \textsc{Quantum Package} strives to allow easy implementation and
experimentation of new methods, while making parallel computation as simple and
efficient as possible on modern supercomputer architectures. Currently, the
code enables, routinely, to realize runs on roughly 2\,000 CPU cores, with tens
of millions of determinants in the reference space. Moreover, we have been able
to push up to 12\,288 cores in order to test its parallel efficiency. In the
present manuscript, we also introduce some key new developments: i) a
renormalized second-order perturbative correction for efficient extrapolation
to the full CI limit, and ii) a stochastic version of the CIPSI selection
performed simultaneously to the PT2 calculation at no extra cost.
