An ab initio quantum mechanical study of hydrogen-bonded complexes of biological interest

A series of hydrogen-bonded dimers are examined via several quantum mechanics (QM) methods, including the Hartree-Fock (HF), second-order Moller-Plesset perturbation (MP2), and local MP2 (LMP2) theories, with different basis sets. The effects of electron correlation, basis set size, and basis set superposition error (BSSE) are systematically analyzed and results are compared with available experimental data. Results from the lower levels of theories examined depend on "error cancelation" and in some cases are not in satisfactory agreement with experiment or high level QM calculations. Higher level methods yield improved results, with the LMP2/cc-pVQZ single point energy evaluation for geometries optimized at the MP2/6-31G* level indicated to be a reliable and economical procedure for accurately determining both the hydrogen-bonding geometries and energies. The importance of the inclusion of electron correlation during geometry optimization is discussed.