Theory of MX Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 112

Fig. 8.4 Phase diagram
expressed by a tetrahedron.
The region named SDW
corresponds to the Mott-
insulator state in this article.
Reprinted from Nasu [ 1 ]
magnon excitations, in the sense of a standard second-order perturbation.
Such further stabilization of the Mott-insulator state beyond the HF level
was also confirmed by a modern density-matrix renormalization group (DMRG)
calculation [ 10 ].
8.1.4 First-Principles Calculation
First-principles calculations are generally very important in discussing materials
especially when they are new and people do not have a rigid picture about them. An
idealistic way of a theoretical approach will be, first, a first-principles calculation
and, next, model calculations on the basis of it. In this sense, the history of
theoretical development on these materials was reverse; first the ideas and
investigations of model calculations, and then first-principles calculations
appeared. I myself think that this is also allowed because such history seems to
indicate a splendid intuition of the people who launched the theory represented by
the models like Eqs. ( 8.1 ) and ( 8.3 ). In fact, the validity of those models was assured
by the first principle calculations done afterward. Alouani et al. performed all-
electron local-density-approximation (LDA) calculations extensively [ 11 - 13 ]. In
their results, it was first confirmed that the most important orbitals around the Fermi
energy were corresponding d z 2 orbitals, but they were strongly hybridized with the
neighboring p z orbitals. In this sense, a single-band model like Eq. ( 8.1 ) should be
interpreted as an effective model in which the orbital at the metal site is a d z 2 orbital
combined with neighboring p z orbitals with appropriate signs, as is the case in the
so-called Zahn-Rice singlet [ 14 ].
They also reported other important properties, for example, the realization of
CDW for Pt systems and that of an undimerized antiferromagnetic insulating state
for a Ni system. Exactly speaking, the latter state found in their calculation is not