Operating in a quantum regime, a molecule-digital 1/2-adder is presented with two NO2 conformational inputs and two tunnel current read-outs. Only five valence molecular orbitals are computing while the 191 others determine the chemical stability and orbital design of the gate. Our gate is based on constructive and destructive electronic interferences through the molecule computing board to vary the electric resistance of the XOR and AND output tunnel junction. A simple tight binding model shows that indeed a minimum of five quantum states is required to design a quantum 1/2-adder. (c) 2007 Elsevier B.V. All rights reserved.