Comment on ‘Molecular structure in non-Born–Oppenheimer quantum mechanics’

2005 ◽  
Vol 408 (4-6) ◽  
pp. 445-447 ◽  
Author(s):  
B.T. Sutcliffe ◽  
R. Guy Woolley
Keyword(s):  
Molecular Structure ◽  
Quantum Mechanics

Subsequent and Subsidiary? Rethinking the Role of Applications in Establishing Quantum Mechanics

2015 ◽  
Vol 45 (5) ◽  
pp. 641-702 ◽  
Author(s):  
Jeremiah James ◽  
Christian Joas
Keyword(s):  
Molecular Structure ◽  
Quantum Mechanics ◽  
Quantum Theory ◽  
Classical Mechanics ◽  
Theory Construction ◽  
Science Pedagogy ◽  
Old Quantum Theory ◽  
The Common ◽  
Complete Quantum

As part of an attempt to establish a new understanding of the earliest applications of quantum mechanics and their importance to the overall development of quantum theory, this paper reexamines the role of research on molecular structure in the transition from the so-called old quantum theory to quantum mechanics and in the two years immediately following this shift (1926–1928). We argue on two bases against the common tendency to marginalize the contribution of these researches. First, because these applications addressed issues of longstanding interest to physicists, which they hoped, if not expected, a complete quantum theory to address, and for which they had already developed methods under the old quantum theory that would remain valid under the new mechanics. Second, because generating these applications was one of, if not the, principal means by which physicists clarified the unity, generality, and physical meaning of quantum mechanics, thereby reworking the theory into its now commonly recognized form, as well as developing an understanding of the kinds of predictions it generated and the ways in which these differed from those of the earlier classical mechanics. More broadly, we hope with this article to provide a new viewpoint on the importance of problem solving to scientific research and theory construction, one that might complement recent work on its role in science pedagogy.

Multi-step modeling of liquid crystals using ab initio molecular packing and hybrid quantum mechanics/molecular mechanics simulations

2017 ◽  
Vol 16 (02) ◽  
pp. 1750012 ◽  
Author(s):  
Hang Hu ◽  
Alejandro D. Rey
Keyword(s):  
Molecular Structure ◽  
Quantum Mechanics ◽  
Raman Spectrum ◽  
Liquid Crystals ◽  
Molecular Mechanics ◽  
Crystal Structures ◽  
Molecular Interactions ◽  
Crystal Packing ◽  
Order Parameters ◽  
Molecular Structures

A density functional theory (DFT) based multi-step simulation method is used to characterize the detailed molecular structure and inter/intra- molecular interactions of two benchmark liquid crystals (LC) 5CB, 8CB and a novel tri-biphenyl ring bent core LC material. The method uses hybrid DFT at the B3LYP/6-31G* level to obtain molecular structure and Raman data. These results are fed to a crystal packing simulation to find possible crystal structures. A pico-second quantum mechanics/molecular mechanics (QM/MM) simulation model is built for the selected structures with lower overall energy as well as optimal density. The stabilized crystal structures are then extended into a super cell, heated and simulated using a mixed force field and nano-second molecular dynamics (MD). The described simulation process sequence provides predictions of molecular Raman spectrum, LC density, isotropic depolarization ratio, ratio of differential polarizability, order parameters, molecular structures, and rotating Raman spectrum of the different mesophases. The Raman spectra, order parameters and depolarization ratios all agree well with existing experimental and previous simulation results. The study of the novel tri-biphenyl ring bent core LC system shows that the ratio of differential polarizability depends on intra-molecular interactions. The findings presented in this manuscript contribute to the on-going efforts to establish links between LC molecular structures and their properties, including optical behavior.

Molecular structure in non-Born–Oppenheimer quantum mechanics

2004 ◽  
Vol 387 (1-3) ◽  
pp. 136-141 ◽  
Author(s):  
Mauricio Cafiero ◽  
Ludwik Adamowicz
Keyword(s):  
Molecular Structure ◽  
Quantum Mechanics

scholarly journals ORBITALS IN GENERAL CHEMISTRY, PART II: MATHEMATICAL REALITIES

Química Nova ◽  
2020 ◽  
Author(s):  
Guy Lamoureux ◽  
John Ogilvie
Keyword(s):  
Molecular Structure ◽  
General Chemistry ◽  
Quantum Mechanics ◽  
Constant Amplitude ◽  
Qualitative Explanation ◽  
Mathematical Functions ◽  
Atomic Orbitals ◽  
Two Factors

In Part II of a three-part series, we discuss two factors absent from textbooks of general chemistry that are important in a discussion of teaching orbitals. First, atomic orbitals are shown systematically to comprise algebraic formulae in coordinates of not one but four sets (spherical polar, paraboloidal, ellipsoidal, spheroconical coordinates). Each formula has its corresponding shape as a surface of constant amplitude; some visual examples are provided. Second, the argument that molecular structure is incompatible with quantum mechanics is presented. Despite the utility of orbitals as mathematical functions in various calculations, they are intrinsically complicated for the traditional purpose of qualitative explanation of molecular structure.

Theory and Quantitative Predictions

2007 ◽  
Author(s):  
James Wei
Keyword(s):  
Molecular Structure ◽  
Quantum Mechanics ◽  
Schrödinger Equation ◽  
Ab Initio ◽  
Molecular Mechanics ◽  
Equilibrium Position ◽  
Schrodinger Equation ◽  
Single Point ◽  
Absolute Temperature ◽  
Structure And Properties

The purpose of this chapter is to review the theories of molecular structure and property relations, to discuss computational methods for prediction of molecular structure and properties, and to discuss some of the properties that can be predicted by computations. Quantum mechanics is the foundation of molecular structure and properties. The position and energy of the electrons around a molecule are determined by solving the Schrödinger equation for a given set of positions of the nuclei of the atoms. There is a lot of powerful and effective computer software that can be used to calculate many of the properties of an isolated single molecule, especially at zero absolute temperature. The starting point is the construction of the sketch of a molecule by connecting atoms with the appropriate bonds. This qualitative sketch does not need accurate values for the bond lengths and angles. To set up the computation, the investigator specifies one of three computation methods: ab initio, semi-empirical, or molecular mechanics. The first and second methods are based on quantum mechanics about a model of the molecule as a number of negatively charged electrons surrounding a collection of positively charged nuclei. The third option of molecular mechanics is based on classical Newtonian mechanics about a model of the molecule as a number of mechanical bonds linking the atoms together, and these bonds can be stretched and bent according to empirical force fields. When either the Schrödinger equation or the Newtonian equation is solved with the initial spatial distribution of nuclei, in what is called the single-point determination, the binding energy of the molecule is obtained. If we make random perturbations of the positions of the various atoms, and repeat the single-point calculations, we can map the energy levels of the molecule in a neighborhood. The most stable or equilibrium position of the molecule is the one with the lowest energy in the neighborhood, and the search for this equilibrium position of the atoms is called geometry optimization. The most rigorous and accurate method of calculation is the ab initio method, which is also the most demanding in computational time and resources, so that it is most often used for smaller molecules.

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