We present the ONIOM-PCM method, which combines the ONIOM (our own n-layered integrated molecular orbital+molecular mechanics) method with the polarizable continuum model (PCM). Four versions of the method have been developed. These schemes differ mainly with respect to the level of coupling between the solute charge distribution and the continuum, which has important consequences for the computational efficiency. Any property that can be calculated by both ONIOM and PCM can also be calculated by the ONIOM-PCM method. In the current paper we use this aspect for the calculation of the derivatives of the energy with respect to the nuclear coordinates to perform geometry optimizations, and the calculation of the nuclear magnetic resonance shielding for solvated molecules. To assess the various versions of the method, we performed ONIOM(B3LYP:Hartree–Fock)-PCM calculations on a merocyanine, H2N(C2H2)3CHO. All four schemes yield results close to the target B3LYP (three-parameter Becke–Lee–Yang–Parr density functional)-PCM, and the method appears to be a promising tool for accurate calculations on large molecules in solution

A thermodynamical cycle is proposed to calculate absolute pK a values for a Brönsted acid in aqueous solution. The solvent (water) was represented by a dielectric using the polarizable continuum model (PCM), and the absolute pK a values of some aliphatic carboxylic acids were computed. The results indicate that the proposed methodology seems to be capable of predicting reasonably good absolute pK a values, although in some cases appreciable deviations are observed, which can be related to neglecting the molecular motion contributions (ΔG Mm) to the solvation energy (ΔG solv).

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Optical rotation values were calculated for the eight most abundant structures of glucose in aqueous solution, following the TD-DFT/GIAO approach for the property and the PCM description for the solvent. The results show that all alpha structures give a large positive contribution to the OR property, while the beta structures give both positive and negative contributions. The good agreement of the calculated OR, obtained as a Boltzmann average of the property of the eight conformers, with experimental data proves the validity of the quantum-mechanical approach and of the solvent modelization.

We present a study on the IR vibrational shifts of the OH stretch mode for water and methanol when passing from gas to liquid phase. Both a purely continuum solvation model (here the polarizable continuum model) and its semicontinuum extensions, obtained explicitly considering small clusters of H-bonded molecules of liquid, have been tested. The definition of such clusters as the new quantum mechanical system to be coupled to a continuum description of the remainder liquid has allowed us to take into account both the “local permanent specific” interaction due to the H-bond and the long-range effects present in all liquids. The analysis mainly focused on harmonic approximations and has been extended to include mechanical anharmonicities in order to have a more reliable comparison with the experiments. Finally, careful attention has been paid to the analysis of the “intrinsic” parameters of continuum models (i.e., the shape and size of the molecular cavity containing the QM system) and to their effects on the vibrations.