DEVELOPMENT OF BOND-LENGTH ALTERNATION IN VERY LARGE CARBON RINGS - LDA PSEUDOPOTENTIAL RESULTS

Carbon rings C-n and infinite chains C-infinity are investigated by mo lecular-orbital and band-structure calculations within the local-densi ty approximation. Carbon rings C-4N (N less than or equal to 8) exhibi t a substantial first-order Jahn-Teller distortion that leads to long/ short (single/triple) bond alternation decreasing with increasing N. R ings C4N + 2 show no alternation (i.e., aromatic behavior is retained) until very large sizes (N greater than or equal to 20). For the infin ite carbon chain uniform Brillouin-zone sampling within even number of points N-s gives bond alternation. An odd number of sampling points g ives no bond alternation for less than N-s = 41. In the large N-s limi t even and odd sampling lead to an upper and lower bound of 0.070(a0) and 0.065a(0) for bond alternation and 0.021-0.090 millihartrees/atom for condensation energy. [S0163-1829(98)51836-X].