The chemist’s concept of molecular structure

Abstract

The concept of molecular structure is fundamental to the practice and understanding of chemistry, but the meaning of this term has evolved and is still evolving. The Born-Oppenheimer separation of electronic and nuclear motions lies at the heart of most modern quantum chemical models of molecular structure. While this separation introduces a great computational and practical simplification, it is neither essential to the conceptual formulation of molecular structure nor universally valid. Going beyond the BornOppenheimer approximation introduces new paradigms, bringing fresh insight into the chemistry of fluxional molecules, proteins, superconductors and macroscopic dielectrics, thus opening up new avenues for exploration. But it requires that our ideas of molecular structure need to evolve beyond simple ball-and-stick-type models.

Extracting elements of molecular structure from the all-particle wave function

“…In order to be able to extract structural information from the allparticle wave function and to recognize classical molecular structural motifs, i.e., bonds and angles, we start out from the probabilistic interpretation of the wave function and calculate marginal probability densities. [44][45][46][47] Then, radial and angular probability densities will be derived from the marginal densities of selected particles, in order to obtain the probability density for the distances and angles of interest. In what follows a short overview of the most relevant literature is given and the density functions are introduced which are used in this work.…”

Molecular structure calculations: A unified quantum mechanical description of electrons and nuclei using explicitly correlated Gaussian functions and the global vector representation

“…4, which also presents one of the central conceptual problems of molecular structure theory: the reconstruction of the classical molecular structure from a fully quantum mechanical description. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Numerical contributions to this question can be found in Refs. 20-29. In recent work, 25,26 we introduced radial and angular density functions to recognize elements of classical molecular structure in the all-particle quantum theory as strong correlation effects for the nuclei.…”

Untitled

“…It has been described as a "felt need," aided and abetted by the Born-Oppenheimer approximation that adiabatically separates electronic and nuclear motion [Wooley, 1978]. Without this separation it is usually argued [e.g., Atkins, 1999], calculations on the quantum states of molecules or understanding their properties, would not be possible (that this is not actually the case is a separate issue [Sukumar, 2009;Dalton et al, 1980]).…”