Molecular quantum similarity periodic system

It is commonly believed that quantum mechanics is sufficient to recreate the chart of the elements. Even if this were so, quantum mechanics gives no justification for the presupposition that a molecular system should resemble the chart of the elements. Thus, the idea that the construction of physical periodic system requires some similarity with the chart of the elements qualifies as an assumption. Consider, for example, diatomic systems. The architectures of Kong (Kong 1982, 1989) have located molecules using two coordinates—a group axis based on the sums of the atomic group numbers, and a period axis based on the sums of the atomic period numbers. Another architecture has the group numbers of the two atoms separate and has their period... 

In recent years there the reduction of chemistiy has been discussed in a variety of ways. Many studies have concentrated on inter-theoretical reduction between theories of chemistry and theories of physics (Bunge, 1982 Primas, 1983). Others have discussed the reduction of chemistry in a naturalistic manner, by examining the question of how some typically molecular properties such as bond angles can be deduced from quantum mechanics in an ab initio fashion or whether the periodic system can be similarly deduced from quantum mechanics (Scerri, 2004). More recently a number of authors have turned to discussing the ontological reduction of chemistry (McLaughlin, 1992 Le Poidevin,... 

In the latter way of looking at the build-up of molecules, the successive addition of electrons to a positively charged system is reminiscent of the manner in which the atoms of the Periodic Table were considered in Chapter 1. Here again there are certain configurations permitted the electron clouds, and these cloud shapes (or probability density functions) can be described using quantum numbers. Such probability density descriptions are called molecular orbitals in analogy to the much simpler atomic orbitals. Although the initial setup and subsequent mathematical treatment for molecules are much more complicated than for atoms, there arise certain similarities between the two types of orbitals. 

With the development of GGA functionals, description of molecular systems with the Kohn-Sham method reached a precision similar to other quantum theory methods. It was quickly shown that the GGA s could also well reproduce the hydrogen bond properties. Short after, liquid water at ambient condition was first simulated by Car-Parrinello MD, with a sample of 32 water molecules with periodic boundary conditions [31]. Since then, many simulations of liquid water at different temperatures and pressures and of water solutions have been performed [32-39]. Nowadays, Car-Parrinello MD has become a major tool for the study of aqueous solutions [40-64]. 

In fact, one of the objectives of the book is to introduce nonexpert readers to modem computational spectroscopy approaches. In this respect, the essential basic background of the described theoretical models is provided, but for the extended description of concepts related to theory of molecular spectra readers are referred to the widely available specialized volumes. Similarly, although computational spectroscopy studies rely on quantum mechanical computations, only necessary aspects of quantum theory related directly to spectroscopy will be presented. Additionally, we have chosen to analyze only those physical-chemical effects which are important for molecular systems containing atoms from the first three rows of the periodic table, while we wiU not discuss in detail effects and computational models specifically related to transition metals or heavier elements. Particular attention has been devoted to the description of computational tools which can be effectively applied to the analysis and understanding of complex spectroscopy data. In this respect, several illustrative examples are provided along with discussions about the most appropriate computational models for specific problems. 

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