A conformational search at the MM3 level was performed for 12-crown-4 (12c4) whereby 180 conformations were predicted. To determine the lowest energy conformations and to get a more accurate energy order of the predicted conformations, geometry optimization was performed for the 180 conformations at the HF/STO-3G level and for the 100 lowest energy conformations, according to HF/STO-3G energy order, at the HF/4-31G and HF/6-31+G* levels. Some of the 100 conformations had equal energies at the three above-mentioned levels and consequently 37 conformations were excluded. Further computations were performed for the 20 lowest energy unique conformations, according to the MP2/6-31+G*//HF/6-31+G* energy order, at the B3LYP/6-31+G*, MP2/6-31+G*//B3LYP/6-31+G*, and MP2/6-31+G* levels. Good agreement was found between the energy order of the conformations at the MP2/6-31+G*//HF/6-31+G* and MP2/6-31+G*// B3LYP/6-31+G* levels and that at the MP2/6-31+G* level, the most accurate level considered in this work. The relative energies of the predicted conformations at the MP2/6-31+G*//B3LYP/6-31+G* level are close to those at the MP2/6-31+G* level, to within 0.1 kcal/mol at most. This is with the exception of only two conformations. This suggests that the cheaper MP2/6-31+G*//B3LYP/6-31+G* level may be used to determine the relative energy order of conformations of larger molecules where the MP2/6-31+G* computations are prohibitively expensive. The closeness of the MP2/6-31+G*//B3LYP/6-31+G* and MP2/6-31+G* relative energies is shown to be a reflection of the closeness of the B3LYP6-31+G* and MP26-31+G* optimized geometries. For the two conformations where the difference of the relative energies was larger than 0.1 kcal/mol, large differences between some of the B3LYP/6-31+G* and MP2/6-31+G* ring dihedral angles were found. The calculated results show that the correlation energy is necessary to obtain an accurate energy order of the predicted conformations. A rationalization of the energy order of some of the predicted conformations in terms of the CH...O interactions is given.