MC-QCISD calculations for proton affinities of molecules and geometries

Multi-coefficient QCISD (MC-QCISD) calculations have been performed to calculate the geometries and energies for neutral and protonated molecules to reproduce the absolute proton affinity scale. The calculated structures at the MC-QCISD//ML level agree very well with experiment and with highly correlated post-Hartree–Fock calculations. The mean absolute deviations from the experimental results of the results at the MC-QCISD//ML and the MC-QCISD//MP2/6-31G(d,p) levels are 4.4 kJ/mol and 4.2 kJ/mol, respectively. We conclude that the MC-QCISD level of theory can provide good accuracy of the proton affinity scale with less cost.