The hydrogen-bond energies of water dimer and water-formaldehyde complexes have been studied using density functional theory (DFT). Basis sets up to aug-cc-pVXZ (X=D, T, Q) were used. It was found that counterpoise corrected binding energies using the aug-cc-pVDZ basis set are very close to those predicted with the aug-cc-pVQZ set. Comparative studies using various DFT functionals on these two systems show that results from B3LYP, mPW1PW91 and PW91PW91 functionals are in better agreements with those predicted using high-level ab initio methods. These functionals were applied to the study of hydrogen bonding between guanine (G) and cytosine (C), and between adenine (A) and thymine (T) base pairs. With the aug-cc-pVDZ basis set, the predicted binding energies of base pairs are in good agreement with the most elaborate ab initio results.
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