  We use the coupled cluster method to high orders of approximation in order to
calculate the ground-state energy, the ground-state magnetic order parameter,
and the spin gap of the spin-1/2 J_1-J_2 model on the square lattice. We obtain
values for the transition points to the magnetically disordered quantum
paramagnetic phase of J_2^{c1}=0.454J_1 and J_2^{c2}= 0.588 J_1. The spin gap
is zero in the entire parameter region accessible by our approach, i.e. for J_2
\le 0.49J_1 and J_2 > 0.58J_1. This finding is in favor of a gapless
spin-liquid ground state in this parameter regime.
