Low-energy conformations of beta-cyclodextrin under anhydrous conditions in the gas phase were investigated by DFT calculations. In one conformation, two homodromic hydrogen bond rings are formed with very short hydrogen bonds at the narrow side of the cyclodextrin ring and a second one at the wider side. Four possibilities for the orientation of the homodromic rings have been taken into account. These 'closed' conformations differ from conformations which have been studied experimentally, forming inclusion complexes with small and medium-sized guest molecules. Their energy is significantly lower than those of the open conformations (Delta E approximate to 44 kJ mol(-1)). (C) 2007 Elsevier B.V. All rights reserved.