  We present an approximate analytical solution to the dynamic equation of two
Ising-coupled qubits with oscillating classical control fields that are
nonperpendicular to the static drift fields. This is a situation that has
recently arisen in some solid-state experiments. With our solution we derive
the analytical expressions for the local invariants as well as the local
rotations needed to isolate a purely nonlocal gate. This determines the set of
parameters that are required to generate any entangling gate. Moreover, we use
our results to describe a recent experimental work on capacitively coupled
singlet-triplet qubits in GaAs and discuss possible differences for a similar
device in silicon.
