  We present a universal set of quantum gate operations based on exchange-only
spin qubits in a double quantum dot, where each qubit is obtained by three
electrons in the (2,1) filling. Gate operations are addressed by modulating
electrostatically the tunneling barrier and the energy offset between the two
dots, singly and doubly occupied respectively. We propose explicit gate
sequences of single qubit operations for Hadamard gate and $\pi$/8 gate, and
the two-qubit controlled NOT (CNOT) gate, to complete the universal set. The
unswitchable interaction between the two electrons of the doubly occupied
quantum dot is taken into account. Short gate times are obtained by employing
spin density functional theory simulations.
