The potential energy surfaces (PESs) of the trimethylene sulfide...(HCl)(n) (n = 1, 2) complexes were explored at the MP2 level of theory by using Pople's and Dunning's basis sets including diffuse functions. Two different axial and equatorial dimer hydrogen-bonded complexes were located on the PESs. The computed geometries are in good agreement with the microwave spectroscopic determination of these two structures. MP4SDTQ//MP2 and QCISD(T)//MP2 single-point calculations were carried out on the significative points on the PESs (minima and transition structures). Comparison among supermolecule uncorrected, counterpoise procedure (CP)-corrected, and BSSE-free complete basis set (CBS) interaction energies suggests that the CP algorithm overcorrects the basis set superposition error (BSSE). However, the supermolecule CP-corrected interaction energies agree rather well with the corresponding BSSE-free symmetry adapted perturbation theory (SAPT) values. The energetic analysis shows that the axial conformer is slightly more stable than the equatorial one, in agreement with the experimental findings. The nature of the interactions in the complexes is analyzed using different theoretical tools: Bader's, NEDA's, Fukui's, and SAPT results are presented and discussed. The interconversion between the two conformers takes place through ring puckering motion involving an energetic barrier of 214-266 cm(-1) (QCISD//MP2 level). This small barrier is consistent with the experimental observation that only the axial conformer is detected when argon is used as a carrier gas. The possibility for a trimer structure to be present in the molecular beam was analyzed from the theoretical viewpoint. Structural and energetic information on the trimethylene sulfide...(HCl)(2) complex from ab initio calculations can be helpful to plan further experimental work to detect such a trimer structure.