Introduction

Abstract

“Supression of details may yield results more interesting than a full treatment. More importantly, it may suggest new concepts. Pure quantum mechanics alone, in all its details, cannot supply a definition of, e.g., an acid or a base or a double bond.” These statements are attributed to E. Schrödinger and they constitute one of the earliest realizations of the necessity of interplay between “quantitative” and “qualitative” quantum theory.¹ In the former case, a preoccupation with the physical significance of mathematical expressions is secondary to obtaining a numerical answer which can be compared with the result of an experiment. Indeed, the mathematical structure of quantum mechanics itself is the actual model. By contrast, “qualitative” theory attempts, through computational tests and reference to experimental facts, to simplify the rigorous equations of “quantitative” theory so that some approximate physical model, which can be routinely applied to chemical problems without the need of explicit calculations, finally emerges. It then follows that, while “quantitative” theory can be elaborated on an ab initio level, “qualitative” theory is always empirical and rests on fundamental assumptions. A better “quantitative” calculation can aid the development of a better “qualitative” physical model, and vice versa. Ultimately, one hopes that the two different theoretical approaches will yield results which are in harmony between them as well as with the results of experimental studies.