The isolated G-C and A-T canonical Watson-Crick DNA base-pairs were optimized by the second-order Moller-Plesset (MP2) method with the 6-311G(d), 6-311G(d,p), 6-311G(2d,p), 6-311G(2df,p) and 6-311G(2df,pd) basis sets. The G-C base pair is found to have an intrinsically nonplanar geometry, while A-T has a planar geometry. The obtained nonplanar geometry of G-C was compared with the experimental as well as previous calculated results to discuss the importance of the nonplanarity of DNA base pairs. The obtained interaction energies between DNA bases vary from -25.29 to -28.88 (G-C) and from -12.34 to -14.94 (A-T) kcal/ mol, depending on the basis sets used here. (C) 2005 Elsevier B.V. All rights reserved.