Field notes, quantum mechanics study

From quantum mechanical calculations. By solving the Schrodinger equation using various approximations, we can obtain forces between different atoms and molecules. These forces can be fitted into a force field. This usually works very well for intra-molecular bonded interactions like bond-stretching, bond-bending, and torsion interactions, but less well for van der Waals interactions. Note that hydrocarbon-zeolite interactions are dominated by van der Waals interactions (see, for example, ref. [24] and chapter 4). Recently, there have been several quantum-mechanical studies of water and methanol in Sodalite [25,26] using the Car-Parrinello technique [27],... 

Here we give an overview of the current status and perspectives of theoretical treatments of solvent effects based on continuum solvation models where the solute is treated quantum mechanically. It is worth noting that our aim is not to give a detailed description of the physical and mathematical formalisms that underlie the different quantum mechanical self-consistent reaction field (QM-SCRF) models, since these issues have been covered in other contributions to the book. Rather, our goal is to illustrate the features that have contributed to make QM-SCRF continuum methods successful and to discuss their reliability for the study of chemical reactivity in solution. 

Where quantum chemical methods have been used to study problems in medicinal chemistry and drug design, it has usually been combined with a continuum approximation [90,107-112], rather than explicit simulation, for the solvent effect. As noted, molecular simulations with an explicit solvent are traditionally performed using classical force fields. The reason for this is obvious quantum mechanical calculations are too time consuming. The coupling of QM with continuum approximations has therefore become convenient. However, the so-called hybrid quantum mechanical and... 

This article contains an interesting note about the structure While molecular structure is a central concept in chemistry and physics, it should be remembered that for an isolated gas phase molecule in field-free space the most information that can be acquired is the average values of structural parameters (i.e., bond distances and angles). This point becomes apparent when molecular calculations are done without the B-O approximation - an almost universal approximation in quantum chemistry. While this approximation is extremely useful and has largely defined the terminology of modern spectroscopy, it also hides some simple quantum mechanical truths about the systems we study. ... 

The purpose of this paper was to illustrate the use of a completely classical model to study the laser enhancement of chemical reactions via a collision induced absorption. It is found that the model is easy to apply to a wide range of collinear and three-dimensional examples. It is interesting to note that the quantum mechanical calculation is in qualitative agreement with the classical calculation in the collinear A + BC examples and that a very simple calculation (as in Fig. 3) can predict the Franck-Condon structure in the quantum mechanical reaction. A further study of three-dimensional systems is necessary to see if the low-energy structure in the reaction probability is always weak or whether this is an artifact of the particular model. Other preliminary calculations have illustrated the laser inhibition of chemical reactions with very small field strengths as well as isotopic effects in these systems. Further work is necessary to explore these interesting possibilities. 

The role of CI2 and monochloroacetic acid in the selective chlorination is a difficult problem to understand from the experimental studies. There are several possible orientations for the reactant, product and promoter molecules inside the complex structure of zeolite-L. In this context, it is pertinent to note that molecular modelling techniques are contributing in considerable amount to understand the reaction mechanisms. Molecular modelling includes force field based calculations [3] such as energy minimisation, Monte Carlo, and molecular dynamics calculations and quantum chemical calculations [4 ] such as EHMO, CNDO/INDO, MOPAC, Hartree-Fock and density functional theory calculations. In this study, we have attempted to apply the combination of molecular graphics, force field calculations and quantum chemical calculations to understand the mechanism of selective chlorination of DCB to TCB over zeolite K-L promoted by monochloroacetic acid. 

Linear response theory [152] is perfectly suited to the study of fluid structures when weak fields are involved, which turns out to be the case of the elastic scattering experiments alluded to earlier. A mechanism for the relaxation of the field effect on the fluid is just the spontaneous fluctuations in the fluid, which are characterized by the equilibrium (zero field) correlation functions. Apart from the standard technique used to derive the instantaneous response, based on Fermi s golden rule (or on the first Bom approximation) [148], the functional differentiation of the partition function [153, 154] with respect to a continuous (or thermalized) external field is also utilized within this quantum context. In this regard, note that a proper ensemble to carry out functional derivatives is the grand ensemble. All of this allows one to gain deep insight into the equilibrium structures of quantum fluids, as shown in the works by Chandler and Wolynes [25], by Ceperley [28], and by the present author [35, 36]. In doing so, one can bypass the dynamics of the quantum fluid to obtain the static responses in k-space and also make unexpected and powerful connections with classical statistical mechanics [36]. 

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