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 (Formula presented.) Depending on the choice of parameters, the approximation has two alternative solutions (Formula presented.) and (Formula presented.) with different patterns of deviations from the real term and its vibrational structure; some typical deviations for (Formula presented.) and (Formula presented.) are described. The difference (Formula presented.) 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 (Formula presented.) which characterizes the positions of vibrational levels in the potential well; it demonstrates the distortion of the vibrational structure of the term (Formula presented.) during the approximation. Based on the data from literature, the functions (Formula presented.) and (Formula presented.) 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.