“…The global minimum 1A shows a 1␣,2-half-chair geometry at A ring; its inversion (e.g., 1A → 1C) is quite easier than that of B and C rings as it costs only about 2 kcal/mol; thus, conformations 1C and 1D, presenting the a 1,2␣-half-chair geometry at A ring, give a small contribution (about 4%) to the overall population. The energy barrier for A ring inversion was also determined by locating the transition state of the conformational interconversion 1A → 1C and its height was 7.04 kcal/mol at the present level of calculations, a value somewhat higher than the barrier determined at the AM1 level (4.6 kcal/mol) [7]. The C17 side chain can assume two energy minimum orientations, the preferred one, in agreement with literature X-ray, CD, and computational data [14][15][16], with a torsional angle τ S of −32 • , the second one, with τ S of −138 • , about 1 kcal/mol higher in energy (see 1A versus 1B or 1C versus 1D).…”