Use a large-ish basis set with diffuse functions, like 6-311+G(d,p), also known as 6-311+G**, or aug-cc-pVTZ. This will be a rather large calculation (120 to 180 carbons), so you may have to cut down the basis set size to double-zeta: aug-cc-pVDZ. If using DFT, be sure to include long-range interactions (Grimme's D3 correction or something of that nature). If you are hoping to get a very accurate result, you may also want to include the counterpoise correction.

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