Codebase release 4.0 for QSpace

This is the documentation for the tensor library QSpace (v4.0) that provides a toolbox to exploit ‘quantum symmetry spaces’ in tensor network states in the quantum many-body context. QSpace permits arbitrary combinations of symmetries including the Abelian symmetries \mathbb{Z}_nℤn and U(1)U(1), as well as all non-Abelian symmetries represented by the simple classical Lie algebras: A_nAn, B_nBn, C_nCn, and D_nDn, or respectively, the special unitary group SU(nn), the odd orthogonal group SO(2n+12n+1), the symplectic group Sp(2n2n), and the even orthogonal group SO(2n2n). The code (C++ embedded via the MEX interface into Matlab) is available (https://bitbucket.org/qspace4u) under the Apache 2.0 license. QSpace is designed as a bottom-up approach for non-Abelian symmetries. It starts from the defining representation and the respective Lie algebra. By explicitly computing and tabulating generalized Clebsch-Gordan coefficient tensors, QSpace is versatile in its operations across all symmetries. At the level of an application, much of the symmetry-related details are hidden within the QSpace C++ core libraries. Hence when developing tensor network algorithms with QSpace, these can be coded (nearly) as if there are no symmetries at all, despite being able to fully exploit general non-Abelian symmetries.