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

Area of Science:

• Quantum Chemistry
• Computational Physics
• Materials Science

Background:

• Full Configuration Interaction Quantum Monte Carlo (FCIQMC) is a high-accuracy quantum chemistry method.
• Efficient implementations are crucial for studying complex molecular systems.
• Existing methods face challenges in scalability and computational cost.

Purpose of the Study:

• To introduce NECI, a state-of-the-art implementation of the FCIQMC algorithm.
• To detail the core functionalities and recent advancements in NECI.
• To provide a scalable and versatile tool for electronic structure calculations.

Main Methods:

• NECI employs a stochastic approach using the Hamiltonian matrix on sparse wave function samples.
• The implementation features powerful parallelization, scaling efficiently to over 24,000 CPU cores.
• It incorporates enhancements like partially deterministic formulations, spin-adapted bases, and transcorrelated Hamiltonians.

Main Results:

• NECI enables the calculation of ground and excited state energies.
• It computes properties via one- and two-body reduced density matrices.
• The software can determine spectral and Green's functions for various systems.

Conclusions:

• NECI offers a highly efficient and scalable solution for FCIQMC calculations.
• Its diverse capabilities make it suitable for both ab initio and model systems.
• The software supports standard input formats and is available under an open-source license.