The ONIOM-PCM method: Combining the hybrid molecular orbital method and the polarizable continuum model for solvation. Application to the geometry and properties of a merocyanine in solution

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.