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We use the coupled cluster method to high orders of approximation in order tocalculate 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 obtainvalues for the transition points to the magnetically disordered quantumparamagnetic phase of J_2^{c1}=0.454J_1 and J_2^{c2}= 0.588 J_1. The spin gapis 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 gaplessspin-liquid ground state in this parameter regime.

The spin-1/2 square-lattice J_1-J_2 model: The spin-gap issue