Atomic orbitals nature

Mulliken s approach is in widespread use for many reasons. First, it is conceptually simple and straightforward. An atomic orbital belongs to the atom on which it is centered. Therefore, any electrons in a particular orbital belong to the atom on which that orbital is centered. This notion is based on a very strong belief that atomic orbital nature persists in molecules. Second, Mulliken population analysis is very easily coded into a computer program and is included in all ab initio and semiempirical packages. When a calculation is run, the user obtains the Mulliken populations by default. Finally, since essentially the birth of modern quantum chemistry use of Mulliken population analysis has gained an almost exalted, if not canonical stature. The Mulliken population analysis has always been reported therefore, it will continue to be used. 

Molecular dipole moments are often used as descriptors in QPSR models. They are calculated reliably by most quantum mechanical techniques, not least because they are part of the parameterization data for semi-empirical MO techniques. Higher multipole moments are especially easily available from semi-empirical calculations using the natural atomic orbital-point charge (NAO-PC) technique [40], but can also be calculated rehably using ab-initio or DFT methods. They have been used for some QSPR models. 

Ihc complete neglect of differential overlap (CNDO) approach of Pople, Santry and Segal u as the first method to implement the zero-differential overlap approximation in a practical fashion [Pople et al. 1965]. To overcome the problems of rotational invariance, the two-clectron integrals (/c/c AA), where fi and A are on different atoms A and B, were set equal to. 1 parameter which depends only on the nature of the atoms A and B and the ii ilcniuclear distance, and not on the type of orbital. The parameter can be considered 1.0 be the average electrostatic repulsion between an electron on atom A and an electron on atom B. When both atomic orbitals are on the same atom the parameter is written , A tiiid represents the average electron-electron repulsion between two electrons on an aiom A. 

In constructing an atomic orbital basis to use in a particular calculation, one must choose from among several classes of functions. First, the size and nature of the primary... 

Natural Atomic Orbital and Natural Bond Orbital Analysis... 

The concept of natural orbitals may be used for distributing electrons into atomic and molecular orbitals, and thereby for deriving atomic charges and molecular bonds. The idea in the Natural Atomic Orbital (NAO) and Natural Bond Orbital (NBO) analysis developed by F. Weinholt and co-workers " is to use the one-electron density matrix for defining the shape of the atomic orbitals in the molecular environment, and derive molecular bonds from electron density between atoms. 

The Natural Atomic Orbitals for atom A in the molecular environment may be defined as those which diagonalize the block, NAOs for atom B as those which diagonalize the D block etc. These NAOs will in general not be orthogonal, and the orbital oecupation numbers will therefore not sum to the total number of electrons. To achieve a well-defined division of the electrons, the orbitals should be orthogonalized. 

NATURAL ATOMIC ORBITAL AND NATURAL BOND ORBITAL ANALYSIS... 

In case the general reader might be wondering about the connection between atomic orbitals and the periodic table, let me address this issue briefly. As mentioned above, in the case of the first paper, the modern explanation for the periodic table is based entirely on the orbital model. It is only by ignoring the approximate nature of the model that the explanation for the periodic system might appear to be full and complete. 

This was a claim that I and several other authors criticized in a number of journals, but unfortunately not in Nature magazine.11 Although the authors of the Orbitals Observed study protested their innocence, it became clear that their claims had been incorrect and exaggerated.12 In a section of the same paper, I discussed the notion that the 4s atomic orbital is occupied before the 3d orbitals. This has subsequently turned out to be incorrect and there is conclusive spectroscopic evidence to the contrary which seemed to have escaped the attention of several authors who have written on this issue, including myself.13... 

The problems which the orbital approximation raises in chemical education have been discussed elsewhere by the author (Scerri [1989], [1991]). Briefly, chemistry textbooks often fail to stress the approximate nature of atomic orbitals and imply that the solution to all difficult chemical problems ultimately lies in quantum mechanics. There has been an increassing tendency for chemical education to be biased towards theories, particularly quantum mechanics. Textbooks show a growing tendency to begin with the establishment of theoretical concepts such as atomic orbitals. Only recently has a reaction begun to take place, with a call for more qualitatively based courses and texts (Zuckermann [1986]). A careful consideration of the orbital model would therefore have consequences for chemical education and would clarify the status of various approximate theories purporting to be based on quantum mechanics. 

This is why I and some others have been agitating about the recent reports, starting in Nature magazine in September 1999, that atomic orbitals had been directly observed. This is simply impossible unless one is using the word "orbital" rather perversely to mean charge density (Scerri, 2000). 

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