Ab initio molecular

From this wave function, one sees how even in the early beginning of molecular quantum mechanics, atomic orbitals were used to construct molecular wave functions. This explains why one of the first AIM definitions relied on atomic orbitals. Nowadays, molecular ab initio calculations are usually carried out using basis sets consisting of basis functions that mimic atomic orbitals. Expanding the electron density in the set of natural orbitals and introducing the basis function expansion leads to [15]... 

In excerpt 3Y, the authors refer to computational results performed with the Gaussian suite of programs, a computational package used to calculate molecular ab initio or semiempirical electronic structure theory. Computational parameters (e.g., the basis set and level of theory) are included in the description. Do not worry if you do not understand the content of excerpt 3Y the language is intended for chemists with a computational or theoretical background. 

The electron distribution around an atom can be represented in several ways. Hydrogenlike functions based on solutions of the Schrodinger equation for the hydrogen atom, polynomial functions with adjustable parameters, Slater functions (Eq. 5.95), and Gaussian functions (Eq. 5.96) have all been used [34]. Of these, Slater and Gaussian functions are mathematically the simplest, and it is these that are currently used as the basis functions in molecular calculations. Slater functions are used in semiempirical calculations, like the extended Hiickel method (Section 4.4) and other semiempirical methods (Chapter 6). Modem molecular ab initio programs employ Gaussian functions. 

Compilation of molecular ab initio polarization propagator calculations that go beyond RPA, 1977-1985"... 

L. F. Pacios, CHECKDEN a computer program to generate ID, 2D and 3D grids of functions dependent on the molecular ab initio electron density, Comput. Biol. Chem. 27, 197-209 (2003). 

Molecular Similarity and QSAR. - In a first contribution on the design of a practical, fast and reliable molecular similarity index Popelier107 proposed a measure operating in an abstract space spanned by properties evaluated at BCPs, called BCP space. Molecules are believed to be represented compactly and reliably in BCP space, as this space extracts the relevant information from the molecular ab initio wave functions. Typical problems of continuous quantum similarity measures are hereby avoided. The practical use of this novel method is adequately illustrated via the Hammett equation for para- and me/a-substituted benzoic acids. On the basis of the author s definition of distances between molecules in BCP space, the experimental sequence of acidities determined by the well-known a constant of a set of substituted congeners is reproduced. Moreover, the approach points out where the common reactive centre of the molecules is. The generality and feasibility of this method will enable predictions in medically related Quantitative Structure Activity Relationships (QSAR). This contribution combines the historically disparate fields of molecular similarity and QSAR. 

In the vast field of ionic crystals doped with impurities, many interesting properties are related to large manifolds of excited electronic states well localised in a small, singular portion of the material, made of the impurity and some neighbour atoms, usually called a cluster, which is under the effect of the rest of the host. Relativistic molecular ab initio methods of the Quantum Chemistry like those described in Sections 2.1.1 and 2.1.2 are, in consequence, applicable to the cluster (or pseudomolecule) when the impurities are heavy elements, provided that the embedding effects of the rest of the host are properly... 

Jankowski, K., Becherer, R., Scharf, P, Schiffer, H., Ahlrichs, R. The impact of higher polarization basis functions on molecular ab initio results. 1. The ground state of F2, J. Chem. Phys. 1985, 82, 1413. 

Polarization Basis Functions on Molecular Ab Initio Results,/. Chem. Pitys.,1985, 82,1413-1419. 

A free database of references for molecular ab initio and density-functional calculations is at qcldb.ims.ac.jp/index.html. 

Both perfect and defective systems are simulated as a finite cluster, i.e., a relatively small cluster is cut out of the bulk structure containing the defect. This kind of strategy is also called the cluster approach The main advantage of this approach is its flexibility first, because it does not assume any translational symmetry and allows us to investigate complex structures as defects in amorphous or disordered solids. Second, high quality standard molecular ab initio codes can be used, which thus allow a high-level quantum-mechanical treatment of the defect zone. 

This will be done by the theoretical simulation techniques described in Section II.B by extracting information from molecular ab initio all-electron calculations. 

The years that have elapsed from the beginning of an extensive use of MEP values from molecular ab initio calculations (1969) have show great changes in the computer hardware and software, and several proposals have become outdated. We shall contain within reasonable limits the quotations, especially of the older literature. 

K. Jankowski, R. Becherer, P. Scharf, H. Schiffer, and R. Ahlrichs, /. Chem. Phys., 82,1413 (1985). The Impact of Higher Polarization Basis Functions on Molecular Ab Initio Results. 1. The Ground State of Fz. 

All of the codes for molecular ab-initio calculations offer at least one set of diffuse functions. Still more extensive basis sets exist, and are described by more comphcated notation. 

The evolution over the last decades of molecular ab initio calculations followed the spectacular methodological and... 

---