Characteristic features of the positron binding structure of some nitrile (–CN functional group) species such as acetonitrile, cyanoacetylene, acrylonitrile, and propionitrile are discussed with the configuration interaction scheme of multi-component molecular orbital calculations. This method can take the electron–positron correlation contribution into account through single electronic–single positronic excitation configurations. Our PA value of acetonitrile with the electronic 6-31++G(2df,2pd) and positronic [15s15p3d2f1g] basis set is calculated as 4.96 mhartree, which agrees to within 25% with the recent experimental value of 6.6 mhartree by Danielson et al. [Phys. Rev. Lett., 2010, 104, 233201]. Our PA values of acrylonitrile and propionitrile (5.70 and 6.04 mhartree) are the largest among these species, which is consistent with the relatively large dipole moments of the latter two systems.

An electron-nucleus Colle-Salvetti-type correlation functional for multicomponent density-functional theory is proposed. We demonstrate that our correlation functional quantitatively reproduces the quantum mechanical effects of protons; the mean absolute deviation value is 2.8 millihartrees for the optimized structures of hydrogen-containing molecules, and the effective potential energy curve of the hydrogen molecule is well reproduced. Since this functional is derived without any unphysical assumption, the strategy taken in this development will be a promising recipe to make new functionals for the potentials of other particles' interactions, such as electron-positron and electron-muon.

Two approaches for the determination of the primary and secondary geometric isotope effect are compared for the exemplary porphyrinoid system porphycene, which has two intramolecular hydrogen bonds. A three-dimensional Born–Oppenheimer potential energy surface is calculated in terms of the symmetric and antisymmetric N–H stretching as well as a low-frequency hydrogen bond vibrational normal mode coordinate. From the respective ground-state nuclear wavefunction the quantum correction to the classical equilibrium geometry is determined. Further, geometry optimization within a full-dimensional multi-component molecular orbital (MC_MO) type calculation, which treats both the electrons and the hydrogen-bonded protons quantum mechanically, is performed. Both approaches yield geometric isotope effects, that is, upon H/D double substitution the hydrogen bonds are weakened and the respective N–N distances increase. In addition the MC_MO calculation gives a H/D isotope effect on the electronic structure, that is, the electronic wavefunction becomes more localized at the deuterium nucleus as compared with the proton case.

Characteristic features of the positron-binding structure of some carbonyl and aldehyde species such as formaldehyde, acetaldehyde, acetone and propionaldehyde are discussed with the configuration interaction scheme of multi-component molecular orbital (MC_MO) calculations. This method can take the electron–positron correlation contribution into account through single electronic–single positronic excitation configurations. Our vertical positron affinity (PA) values of acetaldehyde and acetone with electronic 6-31 + +G(2df,2pd) and positronic [15s15p3d2f1g] basis sets are as 52 and 92 meV, which can be compared to the recent experimental values of 90 and 173 meV by Danielson et al (2010 Phys. Rev. Lett. 104 233201). For formaldehyde we have also found that the PA values are enhanced by including the local vibrational contribution from the vertical PA value of 15 meV to 17, 21 and 25 meV after averaging over the zeroth, first and second vibrational states, respectively, due to the anharmonicity of the potential.