NECI

NECI implements the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm to stochastically compute highly accurate ground and excited electronic energies and properties from many-electron Hamiltonians.

Key Features:

• FCIQMC implementation: Stochastic application of the Hamiltonian matrix to a sparse sampling of wave functions using the Full Configuration Interaction Quantum Monte Carlo approach.
• Parallelization and scalability: Scales efficiently to large high-performance computing deployments, demonstrated on more than 24,000 CPU cores.
• Core state and property calculations: Computes ground and excited state energies and properties via one- and two-body reduced density matrices.
• Spectral and Green's functions: Calculates spectral and Green's functions for ab initio and model systems to probe dynamical and excitation properties.
• Algorithmic enhancements: Incorporates a partially deterministic formulation, operates in a spin-adapted basis, and supports transcorrelated Hamiltonians.
• Integrals and interoperability: Reads and uses integrals in the FCIDUMP file format for integration with quantum chemistry programs.

Scientific Applications:

• Electronic structure of molecules: Accurate determination of molecular electronic structure for ground and excited states.
• High-precision energy and property evaluation: Precise energy calculations and property extraction using one- and two-body reduced density matrices.
• Dynamic and excitation studies: Analysis of spectral and Green's functions for ab initio and model systems to study excitation spectra and dynamics.

Methodology:

Implements FCIQMC by stochastically applying the Hamiltonian to a sparse wavefunction sampling; includes a partially deterministic formulation, spin-adapted basis, support for transcorrelated Hamiltonians, computation of one- and two-body reduced density matrices, and calculation of spectral and Green's functions; reads integrals via FCIDUMP.