LCAO SCF Computations for Ammonia

Abstract

A series of LCAO SCF calculations with Slater-type atomic orbitals were performed for the NH3 molecule over a range of bond lengths and angles (keeping at least C3v symmetry). In addition to a minimal basis set (with two different values for the hydrogen 1s orbital exponent), two types of extended basis sets were examined for the planar and the experimental equilibrium configurations. One of these consisted of a ``double zeta'' set on N and one 1s function on each H (with exponent 1.20), while the other involved a minimal set augmented by four ``bond orbitals''−2s and 2p functions centered on N but having their maximum densities in the bond regions. The results for the minimal basis set are typical for this type of calculation, giving good ionization energies and an inversion barrier height which is twice the experimental value. The extended basis sets, though giving fairly low total energies, resulted in an excessive concentration of charge on the nitrogen atom, with a high dipole moment and a zero, or even small negative, inversion ``barrier''. Direct calculations were also performed for the NH3+ ion in the pyramidal and planar configurations.