# Quantum Package 2.0: An Open-Source Determinant-Driven Suite of Programs

## TL;DR

Quantum Package 2.0 is an open-source software suite for quantum chemistry that enables determinant-driven selected configuration interaction methods, supporting large-scale parallel computations and new methodological developments.

## Contribution

It introduces a flexible determinant-driven framework with new perturbative and stochastic methods, enhancing efficiency and scalability for quantum chemistry calculations.

## Key findings

- Supports runs on thousands of CPU cores with millions of determinants.
- Achieves high parallel efficiency up to 12,288 cores.
- Incorporates new renormalized PT2 correction and stochastic CIPSI selection.

## Abstract

\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.