Molecular Calculations on the Conformation of the HIV-1 Reverse Transcriptase Inhibitor (+)-(S)-4,5,6,7-Tetrahydro-8-chloro-5-methyl-6-(3-methyl-2-butenyl)-imidazo[4,5,1-jk][1,4] benzodiazepine-2(1H)-thione (8-Chloro-TIBO)