Quantum chemistry techniques

Theoretical studies aimed at rationalizing the interaction between the chiral modifier and the pyruvate have been undertaken using quantum chemistry techniques, at both ab initio and semi-empirical levels, and molecular mechanics. The studies were based on the experimental observation that the quinuclidine nitrogen is the main interaction center between cinchonidine and the reactant pyruvate. This center can either act as a nucleophile or after protonation (protic solvent) as an electrophile. In a first step, NH3 and NH4 have been used as models of this reaction center, and the optimal structures and complexation energies of the pyruvate with NH3 and NHa, respectively, were calculated [40]. The pyruvate—NHa complex was found to be much more stable (by 25 kcal/mol) due to favorable electrostatic interaction, indicating that in acidic solvents the protonated cinchonidine will interact with the pyruvate. 

The development of improved ab initio MD algorithms using DFT remains an active area. For one example of work in this area, see T. D. Kiihne, M. Krack, F. R. Mohamed, and M. Parrinello, Phys. Rev. Lett. 98 (2007), 066401. Similar work exists for performing MD using high-level quantum chemistry techniques, as described, for example, in J. M. Herbert and M. Head-Gordon, Phys. Chem. Chem. Phys. 7 (2005), 3629. 

When the limiting conditions of the friction approximation are not valid, e.g., there is strong non-adiabatic coupling or rapid temporal variation of the coupling, there is at present no well-defined first principles method to calculate the breakdown in the BOA. The fundamental problem is that DFT cannot calculate excited states of adsorbates and quantum chemistry techniques, that can in principle calculate excited states, are not possible for extended systems. 

The combination of the cluster model approach and modem powerful quantum chemistry techniques can provide useful information about the electronic structure of local phenomena in metal oxides. The theoretical description of the electronic states involved in local optical transitions and magnetic phenomena, for example, in these oxides needs very accurate computational schemes, because of the generally very large differential electron correlation effects. Recently, two very promising methods have become available, that allow to study optical and magnetic phenomena with a high degree of precision. The first one, the Differ-... 

The most widely used semiempirical quantum chemistry technique for theoretical chemisorption studies is the Extended Hiickel Theory (EHT). The method was first proposed by Hoffmann/95/ in its nonrelativistic form, and by Lohr and Pyykko/96/ and also Messmer/97/ in its relativistic form, based on the molecular orbital theory for calculating molecular electronic and geometric properties. For a cluster the molecular orbitals are expanded as linear combinations of atomic orbitals... 

Baev et al. review a theoretical framework which can be useful for simulations, design and characterization of multi-photon absorption-based materials which are useful for optical applications. This methodology involves quantum chemistry techniques, for the computation of electronic properties and cross-sections, as well as classical Maxwell s theory in order to study the interaction of electromagnetic fields with matter and the related properties. The authors note that their dynamical method, which is based on the density matrix formalism, can be useful for both fundamental and applied problems of non-linear optics (e.g. self-focusing, white light generation etc). 

After a geometry optimization of the extended molecule, using quantum chemistry techniques, is complete, a bias voltage is applied to determine the electronic properties of the extended molecule. During the field calculation, the geometry of the molecule may be kept the same or let it optimize. 

Because the interactions between molecules define the properties of a molecular system, it is important that these interactions capture the correct physical features. For typical molecular distances and for molecular assemblies such as normal liquids, intermolecular interactions are characterized by moderate energies. These energies stem from the interaction of electrons and nuclei on separate molecules and can be explored with quantum chemistry techniques. ... 

When selecting the building blocks, tribute has been paid to the availability of statistical models and the power of present-day computers. With the advent of more powerful computers and novel quantum chemistry techniques, it will be possible to replace the force-field optimization by advanced ab initio approaches and the Hartree-Fock IGLO method used here by schemes that include electron correlation. 

The rapid growth in computer power means that increasingly complex systems can be treated by ab initio quantum chemistry techniques. The vibrational spectrum of the object of interest is often calculated, since this offers a check of whether the structure is a true minimum or a saddle point on the potential surface, as well as being a rigorous test of the results. The amplitudes of vibration are calculated as part of this process and it is relatively straightforward to derive the INS spectrum from these via Equation [9]. In contrast, the infrared and Raman intensities are poorly reproduced for... 

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