Symmetry-adapted coupled-pair approach to the many-electron correlation problem. I. LS -adapted theory for closed-shell atoms

The orthogonally spin- and orbital-symmetry-adapted (LS-coupled) coupled-pair many-electron theory and its extended version involving monoexcited and biexcited cluster coefficients is derived in a form suitable for application to closed-shell atomic systems. A diagrammatic approach, based on second quantization, the time-independent Wick theorem, and the graphical methods of spin algebras, is employed.