  We consider a system of two purely capacitively-coupled singlet-triplet
qubits, and numerically simulate the energy structure of four electrons in two
double quantum dots with a large potential barrier between them. We calculate
the interqubit coupling strength using an extended Hund-Mulliken approach which
includes excited orbitals in addition to the lowest energy orbital for each
quantum dot. We show the coupling strength as a function of the qubit
separation, as well as plotting it against the detunings of the two double
quantum dots, and show that the general qualitative features of our results can
be captured by a potential-independent toy model of the system.
