Molecular systems communication

The work described in this paper is an illustration of the potential to be derived from the availability of supercomputers for research in chemistry. The domain of application is the area of new materials which are expected to play a critical role in the future development of molecular electronic and optical devices for information storage and communication. Theoretical simulations of the type presented here lead to detailed understanding of the electronic structure and properties of these systems, information which at times is hard to extract from experimental data or from more approximate theoretical methods. It is clear that the methods of quantum chemistry have reached a point where they constitute tools of semi-quantitative accuracy and have predictive value. Further developments for quantitative accuracy are needed. They involve the application of methods describing electron correlation effects to large molecular systems. The need for supercomputer power to achieve this goal is even more acute. 

Simply to look at the literature is to convince yourself of the importance that density functional theory (DFT) methods have attained in molecular calculations. But there is among the molecular physics community, it seems to me, a widespread sense of unease about their undoubted successes. To many it seems quite indecent that such a cheap and cheerful approach (to employ Peter Atkins s wonderful phrase) should work at all, let alone often work very well indeed. I think that no-one in the com-mimity any longer seriously doubts the Hohenberg-Kohn theo-rem(s) and anxiety about this is not the source of the unease. As Roy reminded us at the last meeting, the N— representability problem is still imsolved. This remains true and, even though the problem seems to be circumvented in DFT, it is done so by making use of a model system. He pointed out that the connection between the model system and the actual system remains obscure and in practice DFT, however successful, still appears to contain empirical elements And I think that is the source of our present unease. 

Twenty years ago, the concept of coherence in molecular systems was new. In the beginning, and certainly within the chemistry community, the relevance... 

To conclude this article, it is hoped that the discussion of relevant issues and opportunities for chemists presented here will sufficiently stimulate the interest of the chemical community. Their active participation is vital for building our understanding of optical nonlinearities in molecular systems as well as for the development of useful nonlinear optical materials. It is the time now to search for new avenues other than conjugation effects to enhance third-order optical nonlinearities. Therefore, we should broaden the scope of molecular materials to incorporate inorganic and organometallic structures, especially those involving highly polarizable atoms. 

In order to make the fragment decoupling continuous in this generalized description, the input probabilities p i), p°(i ) have to be replaced by the separate distributions reflecting the actual participation of zth AO in the chemical bonds (communications) of the molecule. Therefore, they both have to be related explicitly to the zth row in the conditional probability matrix P(b a) = P(j i), which reflects all communications (bonds) between this orbital input and all orbital outputs / (columns in P(b a)). This link must generate the separate subsystem probabilities p°, when the fragment becomes decoupled from the rest of the molecular system, a - a0, when P(b aa) - P(ba aa)8a/p], where P(ba aa) = [P(a a). Indeed, for the decoupled subsystem a0 = (a, a, ...) only the internal communications of the corresponding block of the molecular conditional probabilities P(ba aa) = P(a a) are allowed. They also characterize the internal conditional probabilities in a0 since... 

R.F. Nalewajski, Multiple, localized and delocalized/conjugated bonds in the orbital-communication theory of molecular systems, Adv. Quant. Chem. 56 (2009) 217. 

R.F. Nalewajski, Chemical bonds from the through-bridge orbital communications in prototype molecular systems, J. Math. Chem. in press. 49 (2011) 546. 

The ability to represent uniquely a chemical compound is a fundamental requirement for storage or transmission of chemical information. We define compounds by their molecular structure, as shown in two-dimensional diagrams or stored in computers. Pronounceable names have been developed for oral and written communication, ranging from the trivial, containing no structural information, to completely systematic names, which can be decoded to yield the original structure. However, the application of systematic nomenclature to complicated structures requires expert knowledge of elaborate systems of nomenclature rules. The use of systematic nomenclature to convey information about the increasingly complex molecular systems handled by today s chemists is both laborious and inefficient. 

The helium pair polarizability increment has been studied extensively [19, 29, 39, 41, 48, 56, 57, 63]. We mention in particular the most recent work by Dacre [48], which includes a careful review of previous results. Systems such as H-H in the and states may be considered as tractable examples representative of various types of real collisional pairs [6,28,54, 55,66,74,85, 144, 145, 147]. Elaborate self-consistent field (SCF) calculations, supplemented by configuration interaction (Cl) corrections are also known for the neon diatom [50]. For atom pairs with more electrons, attempts have been made to correct the SCF data in some empirical fashion for Cl effects [47, 49]. Ab initio studies of molecular systems, such as H2-H2 and N2-N2 have been communicated [16, 18]. 

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