Full vs. constrain geometry optimization in the open–closed method in estimating the energy of intramolecular charge-inverted hydrogen bonds

Abstract

We investigate several variants of the open–closed method to estimate the energy of the intramolecular charge-inverted hydrogen bond in model systems. We show that, depending on whether the geometry of the reference open system is partially or fully optimized, the estimated value of the energy may be from a wide range. It seems that the most reasonable reference system should have geometry parameters optimized but with the simultaneous freezing of those geometry parameters whose relaxation may lead to the significant change in the bonding pattern. Results obtained by means of two other estimating methods are studied. Reasonable estimates of energy values can be obtained only if full geometry optimizations of the molecules being components of the isodesmic reaction do not lead to any significant difference in the bonding pattern comparing with that of the interaction-bonded system. All calculations were performed using the B3LYP/aug-cc-pVTZ approximation.