“…In particular, computer simulation in the frame of HF/6-31G(d) approximation has demonstrated that there are seven minima on the potential energy surface of unsubstituted tetrahydro-1,3-oxazine. The corresponding conformations were axial (global minimum) and equatorial chair and five flexible forms: two 1,4-twist-(1,4-Т), two 2,5-twist-(2,5-Т), and one 3,6-twist-(3,6-Т) [34]. This work aimed to perform conformational analysis of 3-methyltetrahydro-1,3-oxazine I in the frame of the nonempirical HF/6-31G(d) approximation as implemented in HyperChem software [35], the hybrid DFT method PBE/3z, and the RI-МР2/λ2 method (the latter two available in PRIRODA software package [36]).…”