An advantage of the single-particle orbital basis described in this section is the fact that it adapts continuously to both the interwell separation d and the detuning parameter ε. As a result, a very ecient convergence is achieved for any d and ε. The adaptability of our singleparticle orbital basis is illustrated in Fig. . In particular, Fig. (a) displays eigenvalues of the non-trivial auxiliary Hamiltonian H (x) aux [see Eq. (A1)] which implements the TCO connement along the x direction. One observes that for larger values of detuning, these eigenvalues become constant (as was to be expected), and they run parallel to the ε axis.