In this work we calculate interaction and stabilization energies of the the Watson-Crick H-bonded base pairs Guanine-Cytosine and AdenineThymine at the GGA level of DFT using the SIESTA method. Structural optimizations are performed by conjugate gradients without geometrical constraints, allowing both nonzero propeller twist angle between nucleobases and pyramidalization of amino groups. Comparison of the obtained geometries with results from the best ones reported in literature (correlated MP2 level) shows differences of the order of 1% in the intramolecular base distances. On the other hand, proportion between stabilization energies under pair formation is found to have approximately the same value as the reported results, while absolute stabilizations energies are different than the reported ones.