  Using group-theoretical approach we found a family of four nine-parameter
quantum states for the two-spin-1/2 Heisenberg system in an external magnetic
field and with multiple components of Dzyaloshinsky-Moriya (DM) and
Kaplan-Shekhtman-Entin-Wohlman-Aharony (KSEA) interactions. Exact analytical
formulas are derived for the entanglement of formation for the quantum states
found. The influence of DM and KSEA interactions on the behavior of
entanglement and on the shape of disentangled region is studied. A connection
between the two-qubit quantum states and the reduced density matrices of
many-particle systems is discussed.
