Algorithms to build the b K 1 basis and b K 2 1 matrix representation to obtain the b K 2 1 Kramers configuration space functions (KCSFs) via diagonalization will be formally generalized to an arbitrary number of unpaired (open shell) fermions. Effective build up of the b K 2 1 matrix representation will be outlined (including threading and graphical processing unit parallelism) to subsequently obtain the KCSFs via calling external/numerical library routines for diagonalization. The effective build up of the b K 2 1 matrix representation relays on a binary tree search algorithm to allow evaluation the b K i b K j action on a given b K 1 basis vector. The binary tree search avoids the treatment of zero b K 2 1 matrix elements which leads to an exponential acceleration. The implementation ( b K 1 basis creation, b K 2 1 matrix representation, and b K 2 1 matrix diagonalization) will be done in an all in core and all at once manner, hence the available core memory sets the physical limits in practical applications.Memory limitations, sparsity of the b K 2 1 matrix, general case of n fermions in m spinors, and the application of KCSFs will be put into further perspective.additivity, algorithm, binary tree, diagonalization, time reversal 1 | I N TR ODU C TI ON "Zweideutigkeit" (double degeneracy) of electrons/fermions as denoted by Pauli [1] (1925) is an phenomenological feature which had to be accounted for to allow the appropriate physical descriptions and mathematical formulations in quantum mechanics. This was indeed and with charm achieved by Pauli himself when extending the angular momentum concept to define a spin, [1] canonizing the double degeneracy of electronic states. A spin operator had the proper behavior with respect to commutation with the nonrelativistic Hamiltonian, hence spin defines a constant of motion and/or spin momentum conservation. Spin quantization ( b S 2 and b S z ) is one of the essential symmetries employed in the formulation of principal working equations, for example Hartree-Fock method, in advanced electron correlation methods, and in the treatment of matrix elements and open shell systems. This lead to a thorough development of spin algebra to obtain configuration state functions (CSFs), which describe the eigenfunctions of b S 2 and b S z operators. Nevertheless, spin does not commute with the many-fermion relativistic Hamiltonian, and so the "Zweideutigkeit" (double degeneracy) cannot be resolved using the well exploited spin algebra within the relativistic domain. The relativistic reincarnation of double degeneracy was suggested first by Kramers (1930) [2] who enforced a concept of pairs, denoted Kramers pairs. Two years after Wigner (1932) [3] generalized this concept to time-reversal symmetry, that is, the solution of the Schr€ odinger equation is equivalent (double degenerate) to a time reversal transformation between time t and -t. In the static (time independent) picture, the time reversal transformation operator b K has the following form [3][4][5] at the 4component leve...