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<p>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 method-
ological freedoms. The sCI method implemented in 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. 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. </p>
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Quantum Package 2.0: An Open-Source Determinant-Driven Suite of Programs
