Abstract: Armchair single-walled carbon nanotubes (3, 3) with different lengths were studied by AM1 and PM3 methods. We found that the hexagons in the ASWCNTs are distorted. The sum of the square of the orbital coefficients in one layer of the frontier orbitals decreases in zigzag manner from the middle to the edge of the nanotubes. The characteristic infrared absorptions of (3, 3) armchair single-walled carbon nanotubes caculated by AM1 method are between the wave number 1290 and 1645 cm^-1.

Key words: AM1 and PM3 , Armchair single-walled carbon nanotube, Frontier orbital distribution, Infrared spectra