Proper basis set for quantum mechanical studies of potential energy surfaces of carbohydrates

In the current paper we address the optimal selection of basis set for carbohydrates. The following basis sets are used with B3LYP method: 6-31G(d), 6-31G(d,p) 5d, 6-31+G(d,p), 6-311+G(d,p), 6-311++G(d,p), 6-311+G(2d,2p), and 63 311++G(2d,2p), The following molecules were used for the comparison: water, conformers Of alpha-L-fucose, and beta-D-glucose. The 6-31G(d) or 6-31G(d,p) basis set in combination with B3LYP functional provide unacceptably poor results for carbohydrates. (Although the HF/6-31G(d) results are quite good.) The introduction of the diffuse functions on heavy atoms is necessary for good results if B3LYP functional is used (suggested methods are B3LYP/6-31+G(d,p) or 6-311+G(d,p), the latter is closer to basis set limit). The introduction of the diffuse functions (+ +) on hydrogen atoms is not necessary if B3LYP or other density functional method is used for carbohydrates (C) 2002 Elsevier Science B.V. All rights reserved.