This paper is a consolidated presentation of why the author believes the most
promising way forward as regards the mechanism for high-temperature
superconductivity remains with the `chemical negative-U' interpretation,
first sketched out in 1987. Many other mechanisms have been proposed over the
years but none address head on the problem as to why the phenomenon is so
tightly confined to the current small subset of mixed-valent cuprates. Tc
in Hg-1223 under pressure is more than 450% greater than from any
other class of superconductor. By concentrating on the materials side, and how
this controls evolution in real space and k-space properties, a more
tailored outlook can be gained. This includes understanding how cuprates find
themselves in a unique position with regard to chemical bonding, to the
delocalization of charge and to the decay of magnetism. It entails
appreciation of the fine-tuning of metallization which is achieved by the
choice of counter-ions, and the part which the latter play in stabilizing
cuprate valencies and crystal structures. By emphasizing the fundamental role
of the periodic table in this physics I wish to see the `chemical' side to
what is involved given greater consideration. It is clear whether in dealing
with the HTSC materials by transport measurements or nmr or optical properties
or ARPES or neutron scattering that they are `marginal metals'. A formal Fermi
liquid approach has to be pushed to the limit - and beyond. It is a pity that
more high-level cluster calculations, drawing on quantum chemistry, have not
been employed to attack the problem. Nowhere is that effort more greatly
missed than in the need to corroborate formally the shell-closure negative-U
understanding of HTSC adhered to throughout the course of the present work. I
have made double-loading shell-closure fluctuations, with their termination of
pdσ/σ* bonding/antibonding interaction, and the concomitant
collapse in the elevated position of the crucial dx2-y2 state, the lynch
pin to all HTSC events. As the paper describes, the layered square-planar
crystal structure adds its own essential contribution toward the configuring
of a highly restrictive set of conditions. I have attempted to incorporate a
broad enough sweep of experimental results to demonstrate just how restrictive
those conditions are. These any satisfactory interpretation of HTSC has
necessarily and readily to embrace.