The structure and the conformational behavior of the HIV-1 reverse transcriptase inhibitor, 11-cyclopropyl-5,11dihydro-4-methyl-6H-dipyrido[3,2-b2',3'-e][1,4]diazepin-6-one (nevirapine), is investigated by semiempirical (MNDO, AMI and PM3) method, ab initio at the HF/3-21G and HF/6-31G** levels and density functional theory at the B3LYP/6-31G** level. The fully optimized structure and rotational potential of the nitrogen and carbon bond in the cyclopropyl ring were examined in detail. A similar geometrical minimum is obtained from all methods which shows an almost identical structure to the geometry of the molecule in the complex structure with HIV-1 reverse transcriptase. To get some information on the structure in solution, NMR chemical shift calculations were also performed by a density functional theory at the B3LYP/6-31G** level, using GIAO approximation. The calculated 1H-NMR and 13C-NMR spectra for the energy minimum geometry agree well with the experimental results, which indicated that the geometry of nevirapine in solution is very similar to that of the molecule in the inhibition complex. Furthermore, the obtained results are compared to the conformational studies of other non-nucleoside reverse transcriptase inhibitors and reveal a common agreement of the non-nucleoside reverse transcriptase inhibitors. The specific butterfly-like shape and conformational flexibility within the side chain of the non-nucleoside reverse transcriptase inhibitors play an important role inducing conformational change of HIV-1 reverse transcriptase structure and are essential for the association at the inhibition pocket.