NECI: N-Electron Configuration Interaction with an emphasis on state-of-the-art stochastic methods.

Khaldoon Ghanem,Robert J Anderson,Deidre Cleland,Simon D Smart,Oskar Weser,Markus Rampp,Nikolay A Bogdanov,Nike Dattani,George H Booth,Nick S Blunt,Lauretta R Schwarz,Niklas Liebermann,Péter Jeszenszki ,A.y Lozovoi ,F Merz ,Giovanni Li Manni,Werner Dobrautz,Pradipta Kumar Samanta,Kai Guther,James J Shepherd,Eugenio Vitale,Catherine Overy,Hongjun Luo,Ali Alavi,Dongxia Ma

doi:10.1063/5.0005754

Abstract

We present NECI, a state-of-the-art implementation of the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm, a method based on a stochastic application of the Hamiltonian matrix on a sparse sampling of the wave function. The program utilizes a very powerful parallelization and scales efficiently to more than 24 000 central processing unit cores. In this paper, we describe the core functionalities of NECI and its recent developments. This includes the capabilities to calculate ground and excited state energies, properties via the one- and two-body reduced density matrices, as well as spectral and Green's functions for ab initio and model systems. A number of enhancements of the bare FCIQMC algorithm are available within NECI, allowing us to use a partially deterministic formulation of the algorithm, working in a spin-adapted basis or supporting transcorrelated Hamiltonians. NECI supports the FCIDUMP file format for integrals, supplying a convenient interface to numerous quantum chemistry programs, and it is licensed under GPL-3.0.

Highlights

NECI started off in the late 1990s as an exact diagonalization code for model quantum dots1,2 and has evolved into a code to perform stochastic diagonalization of large fermionic systems in finite but large quantum chemical basis sets using theFull Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm.3 This algorithm samples Slater determinant Hilbert spaces using signed walkers by propagation of the walkers through stochastic application of the secondquantized Hamiltonian onto the walker population
We present NECI, a state-of-the-art implementation of the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm, a method based on a stochastic application of the Hamiltonian matrix on a sparse sampling of the wave function
Since the CI-problem is defined by the electronic integrals and subsequent methods depend on the results of the CI-step, namely, the reduced density matrices, it is possible to replace a CI-solver for the existing quantum chemistry code with NECI

Summary

Introduction

NECI started off in the late 1990s as an exact diagonalization code for model quantum dots and has evolved into a code to perform stochastic diagonalization of large fermionic systems in finite but large quantum chemical basis sets using theFull Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm. This algorithm samples Slater determinant (i.e., scitation.org/journal/jcp antisymmetrized) Hilbert spaces using signed walkers by propagation of the walkers through stochastic application of the secondquantized Hamiltonian onto the walker population. Full Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm.. Full Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm.3 This algorithm samples Slater determinant (i.e., scitation.org/journal/jcp antisymmetrized) Hilbert spaces using signed walkers by propagation of the walkers through stochastic application of the secondquantized Hamiltonian onto the walker population. It is similar to the continuum real-space Diffusion Monte Carlo (DMC) algorithm. The nodal structure of the wave function, as encoded by the signed coefficients of the sampled Slater determinants (SDs), emerges from the dynamics of the simulation itself. Being based on an FCI parameterization of the wave function, the FCIQMC method exhibits a steep scaling with the number of electrons and is only suited for relatively small chemical systems compared to those accessible to DMC.

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