Vertical and adiabatic ionization energies of NH−4 isomers via electron propagator theory and many body perturbation theory calculations with large basis sets

Electron propagator theory (EPT) is applied to calculating vertical ionization energies of NH−4 in two stable geometries. The first is a complex of a hydride coordinated to one of the hydrogens of the solvent ammonia molecule, while the other isomer has tetrahedral symmetry. Both structures have been optimized using second order many body perturbation theory with a 6-311++G(d) basis. The former structure is predicted to be 0.42 eV more stable than the latter. A series of third order EPT quasiparticle calculations is carried out to demonstrate basis set saturation. The best EPT calculations give vertical ionization energies of 1.20 eV for the solvated hydride structure and 0.42 eV for the tetrahedral isomer. Both of these calculations are within 0.1 eV of anion photoelectron spectroscopy peaks and suggest that the tetrahedral isomer is responsible for one of these features. After removing an electron from the solvate structure, the complex settles into a shallow van der Waals minimum with a hydrogen atom weakly bound to the ammonia molecule. Calculations on the tetrahedral neutral indicate that there is little relaxation energy along the neutral final state potential energy surface, explaining the sharpness of the observed photoelectron peak.