The orbital vibronic interaction in a D-4h copper oxide cluster Cu4O12H8, w
hich is one of the smallest cluster models for the CuO2 sheet of copper oxi
de compounds, is investigated to increase our knowledge of high-T-c superco
nductors. Only the B-1g, and B-2g vibrational modes can couple to the degen
erate e(u) HOMO of CU4O12H8. The orbital vibronic coupling constants of the
B-1g and B-2g modes to the e(u) HOMO are calculated and analyzed with the
hybrid UB3LYP method that can reasonably describe the anti ferromagnetic in
teractions between neighboring copper ions. The B-1g mode of 191 cm(-1) and
the B-2g modes of 51 and 505 cm(-1) give very large coupling constants. Th
e lowest frequency B-2g mode of 51 cm(-1) most strongly couples to the e(u)
HOMO. The HOMO is localized diagonally in the broken-symmetry antiferromag
netic state, and therefore it is reasonable that this diamond-shaped distor
tion mode significantly couples to the HOMO. (C) 2001 Elsevier Science B.V.
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