  We propose a method of forming matter-wave soliton molecules that is inspired
by the recent experiment of Dris {\it et al.}. In the proposed set-up we show
that if two solitons are initially prepared in phase and with a sufficiently
small separation and relative velocity, a bound pair will always form. This is
verified by direct numerical simulation of the Gross-Pitaevskii equation and by
the derivation of the exact interaction energy of two solitons, which takes the
form of a Morse potential. This interaction potential depends not only on the
separation but also on the relative phase of the solitons and is essential for
an analytical treatment of a host of other problems, such as the soliton gas
and the Toda lattice of solitons.
