Approximation of the electronic terms of diatomic molecules by the Morse function. The role of anharmonicity. II. Simple terms

This article continues the series of works by the authors on the approximation of the electronic terms of a diatomic molecules and their vibrational structure by the Morse formula, which is the simplest anharmonic approximation of the real term U(r). Depending on the choice of parameters, the approximation has two alternative solutions M1(r) and M2(r), with different patterns of deviations from the real term and its vibrational structure; some typical deviations for M1(r) and M2(r) are described. The difference delta(r)=U(r)-M(r) quantitatively shows the changes in the shape of the terms during approximation and can serve as a reference point when choosing its type for solving a specific problem. We introduced an empirical anharmonicity function -2 omega ex(v), which characterizes the positions of vibrational levels in the potential well; it demonstrates the distortion of the vibrational structure of the term U(r) during the approximation. Based on the data from literature, the functions delta(r) and -2 omega ex(v) were constructed for more than 20 molecules and a preliminary classification of electronic terms according to their characteristic features is described. Here we present a group of simple terms with minimal deviations from the Morse shape.