Potential construction via charge perturbation: HCN protonation

Ab initio calculations have been performed for proton attachment to HCN to form the ion HCNH + . The potential for the proton attachment is well reproduced at the level of third order perturbation theory taking the proton's charge as the perturbation. This can be analyzed in terms of electrical properties, e.g. a first-order response with the permanent charge field (moments) of HCN, a second-order response from polarization, and a third-order response from hyperpolarization. The calculations show the expected dominance of these effects at long range and the importance, with each perturbative order of energy correction, of other effects closer in. Then, starting from the electrical analysis, a concise model potential was constructed basically by finding that a relatively simple functional form was needed for the low-order, close-in effects. The utility of this modeling scheme is shown here in the building of a proton bridging potential for HCN–H + –NCH starting from simply combining HCN–H + and H + –NCH potentials.