  We provide an atomistic tight-binding description of a few carriers confined
in ambipolar (n-p) double quantum dots defined in a semiconducting carbon
nanotube. We focus our attention on the charge state of the system in which
Pauli blockade of the current flow is observed [F. Pei et al., Nat.
Nanotechnol. 7, 630 (2012); E. A. Laird et al., ibid 8, 565 (2013)] with a
single excess electron in the n-dot and a single hole in the p-dot. We use the
configuration interaction approach to determine the spin-valley structure of
the states near the neutrality point and discuss its consequences for the
interdot exchange interaction, the degeneracy of the energy spectrum and the
symmetry of the confined states. We calculate the transition energies lifting
the Pauli blockade and analyze their dependence on the magnetic field vector.
Furthermore, we introduce bending of the nanotube and demonstrate its influence
on the transition energy spectra. The best qualitative agreement with the
experimental data is observed for nanotubes deflected in the gated areas in
which the carrier confinement is induced.
