is manuscript is organized as follows. In Sec. II, we brie y describe the main computational methods implemented in P as well as newly developed methods and extrapolation techniques. Section III deals with their implementation. In particular, Sec. III A discusses the computation of the Hamiltonian matrix elements using determinant-driven algorithms, while Sec. III C focuses on the acceleration of the Davidson diagonalization, a pivotal point of sCI methods. In Sec. III D, we focus on the determinant selection step used to build compact wave functions. In a nutshell, the principle is to incrementally build a reference wave function by scavenging its external space for determinants that interact with it. To make this step more a ordable, we designed a new stochastic scheme which selects on the y the more important determinants while the second-order perturbative (PT2) energy is computed using a hybrid stochastic-deterministic scheme. erefore, the selection part of this new stochastic CIPSI selection is virtually free as long as one is interested in the second-order perturbative correction, which is crucial in many cases in order to obtain near full con guration interaction (FCI) results. Section IV brie y explains how we produce spin-adapted wave functions, and Sec. V describes parallelism within P . e e ciency of the present algorithms is demonstrated in Sec. VI C where illustrative calculations and parallel speedups are reported. Finally, Sec. VII discusses the development philosophy of P as well as other relevant technical details. Unless otherwise stated, atomic units are used throughout.