Valence-universal methods

The former approach is referred to as the valence universal (VU) or Pock space MR CC method [51-54] and the latter one as the state universal (SU) or Hilbert space method [55]. In spite of a great number of papers devoted to both the VU and SU approaches, very few actual applications have been carried out since their inception more than two decades ago. Certainly, no general-purpose codes have been developed. This is not so much due to the increased complexity of the MR formalism relative to the SR one, as it is due to a number of genuine obstacles that have yet to be overcome. 

G. A. Gallup, Valence Bond Methods, Cambridge University Press, Cambridge, 2002. 

Y. Mo, Valence Bond Method and Its Applications, Ph.D. Thesis, Xiamen University, Xiamen, 1992. 

Gallup, Gordon A., and ebrary Inc. Valence Bond Methods Theory and Applications. New York Cambridge University Press, 2002. 

The major difference between the Fock-space CC methods and the EOMCC theory lies in the fact that in VUCC approaches different sectors of the Fock space are considered simultaneously (cf. the valence universality condition that VUCC theories must obey [8,12,13]), whereas the EOMCC theory treats different sectors of the Fock space as separate prob-... 

A particular variant of the coupled cluster method, called Fock-space or valence-universal [49,50], gave remarkable agreement with experiment for many transition energies of heavy atoms [51]. This success makes the scheme a useful tool for reliable prediction of the structure and spectrum of superheavy elements, which are difficult to access experimentally. A brief description of the method is given below. A more flexible scheme with higher accuracy and extended applicability, the intermediate Hamiltonian Fock-space coupled cluster approach, is shown in the next section. 

For systems with more than two open shells, it is in general necessary to resort to multireference methods. This section has dealt only with state-specific coupled-cluster methods, also known as state-universal methods or Hilbert space methods, for which a considerable amount of effort has been expended on nonrelativistic multireference methods. The alternative, which is much more suited to multireference problems, is the valence-universal or Fock space method, which has been developed for relativistic systems by Kaldor and coworkers (Eliav and Kaldor 1996, Eliav et al. 1994, 1998, Visscher et al. 2001). 

Lowry, the holder of the new chair in physical chemistry at Cambridge University, spoke in Paris in March 1924 on aspects of the theory of valence, including the electronic theory of valence, and in December 1925 on optical methods of verifying structural chemistry and his own hypothesis of semipolar double bonds in organic compounds. The occasions were meetings of the Societe de Chimique de France and the Societe de Chimie Physique. 62... 

During the early years at McGill University, Whitehead s group concentrated on experimental nuclear quadrupole resonance spectroscopy123 and a variety of n- and all-valence electron semiempirical molecular orbital methods.124 His recent interests have included topics as diverse as density functional theory125 and related topics,126 and molecular models of surfactants. 

During my early years as an assistant professor at the University of Kentucky, I demonstrated the synthesis of a simple quinone methide as the product of the nucleophilic aromatic substitution reaction of water at a highly destabilized 4-methoxybenzyl carbocation. I was struck by the notion that the distinctive chemical reactivity of quinone methides is related to the striking combination of neutral nonaromatic and zwitterionic aromatic valence bond resonance structures that contribute to their hybrid resonance structures. This served as the starting point for the interpretation of the results of our studies on nucleophile addition to quinone methides. At the same time, many other talented chemists have worked to develop methods for the generation of quinone methides and applications for these compounds in organic syntheses and chemical biology. The chapter coauthored with Maria Toteva presents an overview of this work. 

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