  We present NECI, a state-of-the-art implementation of the Full Configuration
Interaction Quantum Monte Carlo 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 24000 CPU cores. In this paper, we describe the core
functionalities of NECI and 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 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.
