-The Roothaan equations have been modified to compute molecular interactions between weakly bonded systems at the SCF level of theory without the basis set superposition error (BSSE). The increase in complication with respect to the usual SCF algorithm is negligible. Calculation of the SCF energy on large systems, such as nucleic acid pairs, does not pose any computational problem. At the same time, it is shown that a modest change in basis-set quality from 3-21G to 6-31G changes the binding energy by about 50% when computed according to standard SCF "supermolecule" techniques, while remaining practically constant when computed without introducing BSSE. Bader analysis shows that the amount of charge transferred between the interacting units is of the same order of magnitude whenperformed on standard SCF wave functions and those computed using the new method. The large difference between the corresponding computed energies is thus ascribed to the BSSE.

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The spin-coupled (SC) theory of molecular electronic structure is introduced as the modern development of classical valence bond (VB) theory. Various important aspects of the SC wave function are described. Attention is particularly focused on the construction and properties of different sets of N-electron spin functions in different spin bases, such as the Kotani, Rumer and Serber. Applications of the SC description to a range of different kinds of chemical problems are presented, beginning with simple examples: the H2 and CH4 molecules. This is followed by the description offered by the SC wave function of more complex situations such as the insertion reaction of H2 into CH2(lA1), the phenomenon of hypervalence as displayed by molecules such as diazomethane, CH2N2, SF6 and XeF2. The SC description of the ground and excited states of benzene is briefly surveyed. This is followed by the SC description of antiaromatic systems such as C4H4 and related molecules.

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Spin-coupled theory is used to investigate the bonding in several hypercoordinate and "normal octet" compounds of main group elements. It is found that d basis functions play much the same qualitative role in hypercoordinate and normal molecules, acting as polarization functions. There are no obvious demarcations in the energy penalty per bond of excluding such functions. No evidence is found to support the traditional notions of spdm hybridization. The spin-coupled approach, also known as the full-GVB model, provides a very clear and simple picture of the bonding in all of the molecules studied. In SF6, for example, the sulfur atom contributes six equivalent, nonorthogonal s p l i k e hybrids, which delocalize onto the fluorine atoms. Each of these two-center orbitals overlaps with a distorted F(2p) function, with the perfect-pairing spin function dominating. The spin-coupled description of PFs is entirely analogous, with remarkably little differentiation between axial and equatorial bonds. A key consideration for all of the hypercoordinate species studied is the polarity of the various bonds. It is suggested that less emphasis than hitherto be placed on the "octet rule" and that the so-called democracy principle be asserted: any valence electron can participate in chemical bonding if provided with sufficient energetic incentive. This idea is pursued for phosphorus and sulphur halides, for XeF2, and for the CHS-, SiHS-, and SiF5-ions. It is argued that there are no significant qualitative differences between the hypercoordinate nature of first-row, second-row, and noble gas atoms in appropriate chemical environments. IntroductionAn extraordinary amount has been written in the past sixty years or so on the question of the nature of the bonding in hypercoordinate (or hypervalent) molecules such as SF6 and PFs.

Cancer represents a serious global health problem, and its incidence and mortality are rapidly growing worldwide. One of the main causes of the failure of an anticancer treatment is the development of drug resistance by cancer cells. Therefore, it is necessary to develop new drugs characterized by better pharmacological and toxicological profiles. Natural compounds can represent an optimal collection of bioactive molecules. Many natural compounds have been proven to possess anticancer effects in different types of tumors, but often the molecular mechanisms associated with their cytotoxicity are not completely understood. The endoplasmic reticulum (ER) is an organelle involved in multiple cellular processes. Alteration of ER homeostasis and its appropriate functioning originates a cascade of signaling events known as ER stress response or unfolded protein response (UPR). The UPR pathways involve three different sensors (protein kinase RNA(PKR)-like ER kinase (PERK), inositol requiring enzyme1α (IRE1) and activating transcription factor 6 (ATF6)) residing on the ER membranes. Although the main purpose of UPR is to restore this organelle’s homeostasis, a persistent UPR can trigger cell death pathways such as apoptosis. There is a growing body of evidence showing that ER stress may play a role in the cytotoxicity of many natural compounds. In this review we present an overview of different plant-derived natural compounds, such as curcumin, resveratrol, green tea polyphenols, tocotrienols, and garcinia derivates, that exert their anticancer activity via ER stress modulation in different human cancers.

The electronic structure and bonding of the N2S2 molecule are studied by the spin-coupled valence bond method. Unusual features are revealed which clarify much of the hitherto puzzling properties of this molecule, leading ultimately to a simple Lewis structure:  The two N atoms of the N2S2 ring bear a substantial negative charge, and the two S atoms, a complementary positive charge. There are four single N−S σ bonds and two lone pairs of π electrons, one pair centered about each N atom. Two further π electrons, one from each of the S atoms, are directly coupled to each other across the ring, giving the molecule the overall character of a singlet diradical. This last is shown to be closely related to the metallic character of the (SN) x polymer.