Ab initio molecular orbital calculations of the infrared spectra of hydrogen-bonded complexes of water, ammonia and hydroxylamine. Part 5.—Effects of convergence criterion and electron correlation on geometry, wavenumber and intensity: the trans-hydroxylamine monomer revisited

Abstract

The equilibrium geometries, vibrational wavenumbers and infrared band intensities of the trans-hydroxylamine monomer have been calculated at the Hartree–Fock level and with the 4–31G and 6–31G** basis sets, using a convergence criterion on the self-consistent-field density matrix of 1 × 10^–9. The results are compared with those derived previously using a convergence limit of 5 × 10^–5 and the differences in the values of the calculated parameters are discussed. The effect of electron correlation is estimated by carrying out the same calculations at the level of second-order Møller–Plesset perturbation theory, using the 6–31G** basis set and convergence thresholds of 1 × 10^–5 and 1 × 10^–9. It is concluded that in order to predict the wavenumbers of low-lying vibrational modes accurately, it is necessary to employ the tightest convergence criterion available, and that significant improvements in the predicted values of all wavenumbers may be achieved by including the effects of electron correlation and by using large, flexible basis sets.