Quantum chemical methodology

Both lines of research are far from being closed and we are confident that their development will contribute useful results. However, without considering future performances I think that it can already be said that it is a good strategy to project the future Quantum Chemical methodology in such a way that the WF is by-passed and the 2-RDM or (better still but more difficult) the l-RDM are directly determined. 

When addressing problems in computational chemistry, the choice of computational scheme depends on the applicability of the method (i.e. the types of atoms and/or molecules, and the type of property, that can be treated satisfactorily) and the size of the system to be investigated. In biochemical applications the method of choice - if we are interested in the dynamics and effects of temperature on an entire protein with, say, 10,000 atoms - will be to run a classical molecular dynamics (MD) simulation. The key problem then becomes that of choosing a relevant force field in which the different atomic interactions are described. If, on the other hand, we are interested in electronic and/or spectroscopic properties or explicit bond breaking and bond formation in an enzymatic active site, we must resort to a quantum chemical methodology in which electrons are treated explicitly. These phenomena are usually highly localized, and thus only involve a small number of chemical groups compared with the complete macromolecule. 

It is difficult to lay down Arm standards of what is an acceptable uncertainty in a quantum chemical result, since this can vary considerably from case to case. It is part of. the quantum chemist s job to decide how accurately a given result must be obtained for his/her purposes, as we shall discuss in this course. However, the accuracy that can be achieved in principle is limited by several fundamental approximations that are made in deriving conventional quantum chemical methodology, and we begin by considering these approximations. 

Jiri Cizek s research program centers on the quantum theory of molecular electronic structure and related developments in quantum chemical methodology, coupled-cluster approaches to many-electron correlation problems,105 large-order perturbation theory,106 dynamical groups and exactly solvable models, lower bounds, and the use of symbolic computation language in physics and in chemistry. 

Relativistic Formulations. - Over the past twenty years or so, we have witnessed a continued and growing interest in relativistic quantum chemical methodology and the associated computational algorithms which facilitate... 

Some aspects of computational quantum chemistry applied to the analysis of the electronic structure of polymers are reviewed in connection with the timely trends observed in their electrical and optical properties. The paper is organized as follows after an introduction (Section 36.1), the basic theory of the quantum chemical methodologies as applied to periodic chains is summarized (Section 36.2). Several fields of applications are then presented photoelectron spectra (Section 36.3), conducting and semiconducting conjugated polymers (Section 36.4), hnear and non-linear optical properties (Section 36.5) and the role of charge transfer in organic chains (Section 36.6). Possible developments for the near future are also sketched. 

Hydrodcsulfurization or HDS, is a well-known process for the removal of sulfur atoms from sulfur-containing molecules such as thiophene, benzothiophene, etc. By far the most widely used HDS catalysts consist in a mixture of Ni or Co with MoS supported over silica. The MoS> catalyst has been one of the most studied catalysts. It has been analyzed using different quantum chemical methodologies, from the periodic methods such as VASP up to the cluster model method. The actual literature contains a respectable amount of experimental... 

A variety of ab initio quantum chemical methodologies has been used to compute the internal energy variation, AE°, and the zero-point correction energies AZPE, needed to calculate gas-phase acidities and proton affinities. 

Tetrasilabutadiene 1 is the first example of a compound with two Si=Si double bonds, only separated by a Si-Si single bond. The question arises to which extent conjugation between the two Si=Si double bonds in 1 is possible. We applied quantum chemical methodologies to answer this fundamental question. Tetrasilabutadiene 10 was used as model for 1 and was compared with 1,3-butadiene, 1,4-disila-1,3-butadiene 12 and 2,3-disila-1,3-butadiene 13 [11]. 

At the end of the 1980s, improved ab initio quantum chemical methodologies became available which were suitable for computing, in a balanced way, excited- and ground-state potential-energy surfaces. In particular, the ab initio CASSCF method had an analjAical gradient which could be... 

Thermal organic reactions are often classified in terms of the molecular and electronic structure of their transition state or reactive intermediate (which is often taken as a model of the transition state). Thus, for instance, one has the Sn2 transition state for concerted bimolecular nucleophilic substitution reactions one has the E2 and Ei transition states in elimination reactions, etc. Given the transient nature of the transition states, the use of quantum chemical methodologies is essential for the determination of their detailed geometrical and electronic structure. Furthermore, the computation of the associated transition vectors provides information on the reactive mode... 

---