  We have adopted the multistep shell model in the complex energy plane to
study nuclear excitations occurring in the continuum part of the spectrum. In
this method one proceeds by solving the shell model equations in a successive
manner. That is, in each step one constructs the building blocks to be used in
future steps. We applied this formalism to analyze the unbound nuclei
$^{12,13}$Li starting from the one-particle states in $^{10}$Li and
two-particle states in $^{11}$Li. In the former case the excitations correspond
to the motion of three particles partitioned as the product of a one-particle
and two-particle systems. The ground state of $^{12}$Li is thus calculated to
be an antibound (virtual) state. In the four-particle system $^{13}$Li the
states can be constructed as the coupling of two correlated pairs. We found
that there is no bound or antibound state in $^{13}$Li.
