Valence instability in a dimer of two-orbital system:: possible charge separation due to 'negative U' effect

A novel mechanism of charge separation in a dimer of two-orbital molecular system is proposed. The effective electron-electron repulsive energy in the dimer, U-eff, has been estimated by means of valence-bond (VB) model calculations for the ground state of the dimer. It has been found that the HOMO-LUMO interplay leads to negative U-eff in the strong dimerization condition, indicating spontaneous separation of charges on dimers (2 dimer(-) --> dimer(0) + dimer(2-)). Possible relevance to experimental features in some [Pd(dmit)(2)] salts (dmit = 1,3-dithiol-2-thione-4,5-dithiolate) is discussed. (C) 2004 Elsevier B.V. All rights reserved.