Ab initio molecular orbital theory with the 6-31G(d), 6-31G(d,p), 6-31+G(d), 6-31+G(d,p), and 6-311+G(d,p) basis sets and density functional theory (BLYP, BP86) including the hybrid density functionals B3LYP, B3P86, and B3PW91 have been used to investigate stereoelectronic hyperconjugative interactions and relative energies of the chair, 1,4-twist, and 2,5-twist conformers of 1,3-dithiane (1,3-dithiacyclohexane). The HF/6-31G(d) energy difference (DeltaE) between the chair conformer and the 1,4-boat transition state was 5.53 kcal/mol, and the B3LYP/6-311+G(d,p) energy difference between the chair conformer and the 2,5-boat transition state was 5.42 kcal/mol. Intrinsic reaction coordinate (IRC, minimum energy path) calculations have been used to connect the enantiomers of the 1,4-twist conformer via the 2,5-boat transition state. The B3LYP/6-311+G(d,p) calculated energy difference between the 1,4-twist conformer and the 2,5-boat transition state was 0.80 kcal/ mol. The HF/6-31G(d) energy difference (DeltaE) between the chair conformer and the 2,5-twist conformer was 4.24 kcal/mol, and the 2,5-twist conformer was 0.48 kcal/mol lower in energy than the 1,4-twist conformer. The chair-1,4-twist free energy difference (DeltaG(c-t)(o)) is 4.42 kcal/mol, and the chair-2,5-twist DeltaG(c-t)(o) is 4.27 kcal/mol. IRC calculations connected the chair conformer and 2,5-twist conformer on the potential energy surface by a path that passes through the transition state [TS-1](double dagger) between them. IRC reaction path computations also connected transition state [TS-2](double dagger) to the chair conformer and the 1,4-twist conformer. The transition state [TS-2](double dagger) between the chair and the 1,4-twist conformers is 9.89 kcal/mol higher in energy than the chair conformer, and the transition state [TS-1](double dagger) between the chair and the 2,5-twist conformers is 10.44 kcal/mol higher in energy than the chair conformer. The C2-Hax, C4-Hax, and C6-Hax bond lengths are longer than the corresponding C-Heq bond lengths in the chair conformer (LPs --> sigma*(C-Hax), sigma (C-Hax) --> sigma*(C-Hax)). In contrast, the C5-Heq bond in the chair conformer is longer than the C5-Hax bond (sigma(S-C) --> sigma*(C5-Heq), W effect, or homoanomeric LPs --> sigma*(C5-Heq)). The importance of geometrical considerations in stereoelectronic hyperconjugative interactions is shown in the twist conformers and transition states of 1,3-dithiane. Unlike the chair conformer, in the 2,5-twist conformer, the respective C-H bond lengths at each carbon are equal and the C2-Hiso bond lengths are shorter than the C5-Hiso bond lengths. In the 1,4-twist conformer, the respective C-H bond lengths at C2, C4, and C6 are equal and the C5-Heq bond length is longer than the C5-Hax bond length (sigma(S-C) --> sigma*(C5-Heq), W effect, or homoanomeric LPs --> sigma*(C5-Heq)).