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

Abstract

Quantum chemistry is a discipline which relies heavily on very expensive numerical computations. The scaling of correlated wave function methods lies, in their standard implementation, between and , where N is proportional to the system size. Therefore, performing accurate calculations on chemically meaningful systems requires (i) approximations that can lower the computational scaling and (ii) efficient implementations that take advantage of modern massively parallel architectures. 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 multireference 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 freedom. 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.

A Mountaineering Strategy to Excited States: Highly-Accurate Energies and Benchmarks for Medium Size Molecules

“…Because SCI methods are less widespread than the other methods mentioned in the Introduction, we shall detail further their main features. All the SCI calculations have been performed in the frozen-core approximation with the latest version of QUANTUM PACKAGE 78 to produce a smoother and faster convergence of the SCI energy towards the FCI limit. For the largest systems, an additional iteration is sometimes required in order to obtain better quality natural orbitals and hence well-converged calculations.…”

“…At each iteration, the variational (or reference) space is enlarged with new determinants.CIPSI can be seen as a deterministic version of the FCIQMC algorithm developed by Alavi and coworkers 79. We refer the interested reader to Ref 78. where our implementation of the CIPSI algorithm is detailed.…”

Advanced Computation Method for Double Core Hole Spectra: Insight into the Nature of Intense Shake-up Satellites

“…The NOTA+CIPSI calculations were done with the software quantum package 2.0 [46]. We implemented in this software all tools necessary to compute core hole spectra including the routines to use non-orthogonal MO basis sets.…”

“…The variant used in this work is based on an iterative selection procedure using a second-order Epstein-Nesbet perturbation theory [43,44] criterion and a semi-stochastic approach [45] to compute the perturbative second-order (PT2) energy (see Technical Details bellow and Ref. 46). We note that this CIPSI algorithm is tuned to balance the selection over all of the states considered.…”

Stochastic perturbation theory in a limited configuration space

“…6,7 SCI has renewed attention for further improvements in recent years. [8][9][10][11][12][13][14][15][16][17][18][19] Other modern quantum techniques, such as density matrix renormalization group (DMRG), [20][21][22] have been developed to tackle problems in large Hilbert spaces that were out of reach in the past. More recently, we have introduced the full coupled-cluster reduction (FCCR) approach to demonstrate that the selected coupled-cluster is promising for a balanced treatment of dynamic and nondynamic correlation effects on the same footing.…”