  We present KITE, a general purpose open-source tight-binding software for
accurate real-space simulations of electronic structure and quantum transport
properties of large-scale molecular and condensed systems with tens of billions
of atomic orbitals (N~10^10). KITE's core is written in C++, with a versatile
Python-based interface, and is fully optimised for shared memory multi-node CPU
architectures, thus scalable, efficient and fast. At the core of KITE is a
seamless spectral expansion of lattice Green's functions, which enables
large-scale calculations of generic target functions with uniform convergence
and fine control over energy resolution. Several functionalities are
demonstrated, ranging from simulations of local density of states and
photo-emission spectroscopy of disordered materials to large-scale computations
of optical conductivity tensors and real-space wave-packet propagation in the
presence of magneto-static fields and spin-orbit coupling. On-the-fly
calculations of real-space Green's functions are carried out with an efficient
domain decomposition technique, allowing KITE to achieve nearly ideal linear
scaling in its multi-threading performance. Crystalline defects and disorder,
including vacancies, adsorbates and charged impurity centers, can be easily set
up with KITE's intuitive interface, paving the way to user-friendly large-scale
quantum simulations of equilibrium and non-equilibrium properties of molecules,
disordered crystals and heterostructures subject to a variety of perturbations
and external conditions.
