The Capabilities of Computational Chemistry

At the beginning of the 1990s, the factors in Table 1.1 were generally beyond the capability of computational chemistry methods to predict reliably. However, as the decade unfolded, computational chemists and other scientists... 

Figure 1 The capabilities of computational chemistry underlie computer-assisted molecular design capabilities, which in turn underlie die examples of chemical products reaching the market.

Computer simulations of ionic liquids have been reported in increasing numbers during the past few years. Some of the modeling work has been focused on the development of force field parameters, specific to an ionic liquid or an ionic liquid family [4-10]. Structural, dynamic, electric, and thermodynamic properties of several pure ionic liquids have been simulated [10-12] using these force field tools, and the solvation of small solutes in ionic liquids has also been investigated [13-19]. More recently, an ab initio molecular dynamics study has also been published [20], pushing the capabilities of computational chemistry to its current limits. 

An examination of the other articles in this text serves as an excellent illustration of the diverse analytical methods that have been successfully applied to lignocellulosic materials. The practitioners of wood chemistry have rapidly assimilated and adapted modern instrumental chemistry to their specific problems. In contrast, the techniques of computational chemistry have not been widely used in such an environment. The current paper will attempt to describe the capabilities, opportunities, and limitations of such an approach, and discuss the results that have been reported for lignin-related compounds. 

With this introduction to the methods of computational chemistry, the attention of this paper will now turn to specific applications related to the chemistry of lignin. As promised at the beginning of this paper, the discussion will address not only the capabilities and opportunities that may accrue from this type of research, but will also consider the limitations of the techniques. 

During the past 10 - 15 years, Kohn-Sham density functional theory has been a major factor in a dramatic expansion of the scope of computational chemistry and its capability for treating systems of practical importance [45-51]. Density functional methodology includes electronic correlation, so that the energies are more accurate than Hartree-Fock however the Kohn-Sham formalism is similar to the latter, as are therefore the demands upon computer resources. It is therefore feasible to treat relatively large systems at a reasonably high (post-Hartree-Fock) level. 

The earliest applications of quantum chemistry were targeted toward molecules in the gas phase. This was in part due to an interest in such an environment which allows researchers to focus on the intrinsic properties of the molecule of interest. But also, since much of practical chemistry takes place in solution of some sort, this environment presents an important avenue of inquiry as well. However, such solvated systems were typically out of reach of quantum calculations. For one thing, the inclusion of a number of solvent molecules into the calculations would commonly take the system beyond the capabilities of computers at the time. Secondly, solvent... 

What is the prognosis for the future of computational chemistry Computing hardware is nowadays stellar. Even commonplace computers are capable of handling calculations of systems containing thousands of particles and simulation times in the range of nanoseconds on a single processor. As the power of modem computers continues to grow, it is only a matter of time before computational techniques are used more routinely for problems of interest to supramolecular chemists. Calcula-... 

The proliferation of sophisticated instruments which are capable of rapidly producing vast amounts of data, coupled with the virtually universal availability of powerful but inexpensive computers, has caused the field of chemometrics to evolve from an esoteric specialty at the perhiphery of Analytical Chemistry to a required core competency. 

Solubility modelling with activity coefficient methods is an under-utilized tool in the pharmaceutical sector. Within the last few years there have been several new developments that have increased the capabilities of these techniques. The NRTL-SAC model is a flexible new addition to the predictive armory and new software that facilitates local fitting of UNIFAC groups for Pharmaceutical molecules offers an interesting alternative. Quantum chemistry approaches like COSMO-RS [25] and COSMO-SAC [26] may allow realistic ab-initio calculations to be performed, although computational requirements are still restrictive in many corporate environments. Solubility modelling has an important role to play in the efficient development and fundamental understanding of pharmaceutical crystallization processes. The application of these methods to industrially relevant problems, and the development of new... 

Biochemistry and chemistry takes place mostly in solution or in the presence of large quantities of solvent, as in enzymes. As the necessary super-computing becomes available, molecular dynamics must surely be the method of choice for modeling structure and for interpreting biological interactions. Several attempts have been made to test the capability of molecular dynamics to predict the known water structure in crystalline hydrates. In one of these, three amino acid hydrates were used serine monohydrate, arginine dihydrate and homoproline monohydrate. The first two analyses were by neutron diffraction, and in the latter X-ray analysis was chosen because there were four molecules and four waters in the asymmetric unit. The results were partially successful, but the final comments of the authors were "this may imply that methods used currently to extract potential function parameters are insufficient to allow us to handle the molecular-level subtleties that are found in aqueous solutions" (39). 

The accuracy achieved through ab initio quantum mechanics and the capabilities of simulations to analyze structural elements and dynamical processes in every detail and separately from each other have not only made the simulations a valuable and sometimes indispensable basis for the interpretation of experimental studies of systems in solution, but also opened the access to hitherto unavailable data for solution processes, in particular those occurring on the picosecond and subpicosecond timescale. The possibility to visualize such ultrafast reaction dynamics appears another great advantage of simulations, as such visualizations let us keep in mind that chemistry is mostly determined by systems in continuous motion rather than by the static pictures we are used to from figures and textbooks. It can be stated, therefore, that modern simulation techniques have made computational chemistry not only a universal instrument of investigation, but in some aspects also a frontrunner in research. At least for solution chemistry this seems to be recognizable from the few examples presented here, as many of the data would not have been accessible with contemporary experimental methods. 

The most important feature of the ab initio quantum chemistry computation is the capability of describing the breaking and... 

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