Nessi: The Non-Equilibrium Systems Simulation Package

The nonequilibrium dynamics of correlated many-particle systems is of interest in connection with pump-probe experiments on molecular systems and solids, as well as theoretical investigations of transport properties and relaxation processes. Nonequilibrium Green's functions are a powerful tool to study interaction effects in quantum many-particle systems out of equilibrium, and to extract physically relevant information for the interpretation of experiments. We present the open-source software package NESSi (The Non-Equilibrium Systems Simulation package) which allows to perform many-body dynamics simulations based on Green's functions on the L-shaped Kadanoff-Baym contour. NESSi contains the library libcntr which implements tools for basic operations on these nonequilibrium Green's functions, for constructing Feynman diagrams, and for the solution of integral and integro-differential equations involving contour Green's functions. The library employs a discretization of the Kadanoff-Baym contour into time N points and a high-order implementation of integration routines. The total integrated error scales up to O(N-7), which is important since the numerical effort increases at least cubically with the simulation time. A distributed-memory parallelization over reciprocal space allows large-scale simulations of lattice systems. We provide a collection of example programs ranging from dynamics in simple two-level systems to problems relevant in contemporary condensed matter physics, including Hubbard clusters and Hubbard or Holstein lattice models. The libcntr library is the basis of a follow-up software package for nonequilibrium dynamical mean-field theory calculations based on strong-coupling perturbative impurity solvers.Program summaryProgram Title: NESSiCPC Library link to program files: http://dx.doi.org/10.17632/973crf9hgd.1Licensing provisions: MPL v2.0Programming language: C++, pythonExternal routines/libraries: cmake, eigen3, hdf5 (optional), mpi (optional), omp (optional)Nature of problem: Solves equations of motion of time-dependent Green's functions on the Kadanoff-Baym contour.Solution method: Higher-order solution methods of integral and integro-differential equations on the Kadanoff-Baym contour. (c) 2020 Published by Elsevier B.V.