Pauling’s resonating valence bond theory of metals: some studies on lithium clusters

Abstract

We report for the first time fully ab initio valence bond (VB) calculations with explicit use of the unsynchronized resonance structures introduced by Pauling [1]. We show that resonance involving these structures largely determines the stability and conformation of the Li 4 cluster and plays a central role in a VB explanation of the 3-center bonds in planar alkali clusters. The theory can make qualitative predictions on the behaviour of general metallic clusters, and can relate stability and conformation to electronic structure, thus indicating the origin of magic numbers. This first ab initio test of Pauling's resonating VB theory confirms the importance of the metallic orbital and the covalent character of the metal-metal bond. PACS: 31.10.+z; 31.20.Rx; 36.40+d; 71.10+x I IntroductionHistorically, two main approaches have been used to describe chemical bonds in molecules and solids: the molecular orbital (MO) and valence bond (VB) theories. For metals, however, only the band theory (the crystal analogue of the MO approach) has been fully accepted and widely used in discussing electronic structure and properties. The reason seems to be not only the technical difficulty of making VB calculations but also the long-standing belief, going back to Bragg [2], that localized covalent bonds cannot be present in metals. Only Pauling, many years ago, put forward an alternative VB theory [1], albeit based on mainly empirical arguments. More recently, statistical arguments have been advanced in support of these ideas [3] and have been used in the interpretation of the superconductivity of some copper oxide compounds [4].In this paper we first sketch Pauling's approach, also known as resonating valence bond (RVB) theory, and briefly summarize some of the conclusions reached using the more orthodox MO methods. In following sections we outline our (non-empirical) RVB calculations, making comments on various aspects of the VB approach and its implementation, introduce an RVB explanation of some properties of metal clusters, and finally discuss the implications of our results.The main premises of Pauling's approach are as follows: 1. In metals the atoms possess an orbital free to receive electrons, the metallic orbital, in addition to those occupied by bonding electrons and unshared electron pairs. 2. In addition to the synchronized resonance among the usual covalent Kekulé type structures (which always show bonds between alternative pairs of atoms and in which displacement of one link implies the simultaneous displacement of another) there can be unsynchronized resonance in which the same atom, by invoking the metallic orbital, can acquire two electronpair bonds i.e. gain a second link without losing the first: the new structures will be referred to as Pauling structures. 3. By the principle of electroneutrality net charges higher than ±1 will not be found on any atom.Pauling used empirical and pictorial arguments to suggest that unsynchronized resonance would not only confer increased stability on a metallic lattice but would...

Charge transfer on the metallic atom-pair bond and the bond energy

“…25 Diffusion quantum Monte Carlo simulations of Li clusters show that RVB pairing correlations between electrons provide a substantial contribution to the cohesive energy. 26 Elsewhere, 27 valence-bond studies of aspects of electron conduction along a linear chain of lithium atoms have been reported.…”

Supercondutividade: um século de desafios e superação

“…A idéia intuitiva da ressonância não-sincronizada foi empregada por Pauling no estudo de uma série de processos que envolvem a transferência de elétron [91,93,95], incluindo a supercondutividade [97][98][99][100][101]. Diversos trabalhos atualmente confirmam as idéias de Pauling, as quais vêm sendo aplicadas ao estudo de casos variados como carcinogênese química [102], mecanismo de interação de moléculas com superfícies metálicas [103], condutividade elétrica no lítio [104], formação e estabilidade do O 4 [105], magnetismo [106], estrutura de metais, ligas e compostos intermetálicos [107], condutividade elétrica em sistema orgânicos 1D [108] e não menos interessante, a supercondutividade [109].…”

“…Mesmo não tendo alcançado popularidade equivalenteà de outras teorias, a amplitude de aplicações da teoria RVB vêm produzindo resultados satisfatórios em diferentesáreas, o que ajuda a corroborar sua proposta [97][98][99][100][101][102][103][104][105][106][107][108][109]. A aplicação dos conceitos da teoria RVB ao estado supercondutor abre um novo leque de possibilidades em pesquisa, cujo potencial ainda não tem sido plenamente explorado.…”

Gaussian integral transforms of unconventional functions: contracted basis functions appropriate to molecules and metallic clusters

“…The atomic orbitals are then optimized in order to minimize the total energy. We do not intend here to compare them with CI calculations of the literature since we have shown in a previous work [24] that, for metallic systems, Pauling's RVB theory [25] must be used to properly account for the correlation. This means that structures other than the common covalent and ionic Kekule`ones should appear in the VB wavefunction.…”