The conformations of the HIV-1 reverse transcriptase inhibitor 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT) are calculated by semiempirical and mainly by ab initio methods in order to estimate the potential for the rotation around the carbon sulfur single bond. The results are compared to X-ray structures of HEPT associated to the HIV-1 reverse transcriptase. The NMR spectra of the compound are calculated to obtain some information about its structure in solution. The structure of HEPT in the complex is analysed to study the intermolecular interactions between the inhibitor and the surrounding protein, which determine the geometry of the inhibition complex.