An Assessment of Alternative Low Level Calculation Methods for the Initial Selection of Conformers of Diastereomeric Esters

Table 2

Determination of the cut-off conformer (COFC) energy^a (given in bold typeface) within a series of optimized with LLMs and HF/STO-3G conformers (Steps 1 and 2 and Step 4 (the first level), resp., Figure 2).

Conformer	RefM2	RefM3	HF	UFF	Dreiding	AM1	PM3	PM6	MM3	MM+
a	0.00	0.40	0.09	0.80	0.99	1.43	0.35	3.37	0.03	0.42
b	0.09	0.00	0.09	0.60	0.64	1.31	1.56	3.24	0.00	0.32
c	0.56	0.41	0.27	1.13	0.95	1.40	0.60	4.29	3.05	0.44
d	0.57	0.57	0.89	2.36	1.87	1.43	1.48	3.27	0.90	1.09
e	0.59	0.73	0.86	2.13	2.03	1.39	1.48	3.16	0.89	0.91
f	0.60	0.52	0.24	1.31	1.44	1.26	1.50	4.18	3.09	0.78
g	0.67	0.91	0.91	2.20	2.16	1.45	1.45	3.16	0.99	0.68
h	0.69	1.00	0.88	2.32	2.23	1.35	1.44	3.27	0.96	0.58
i	0.72	0.74	0.01	0.29	1.06	2.11 ^a	1.56	4.81	2.99	0.20
j	0.82	0.87	0.00	0.00	0.00	2.00	1.22	4.13	0.29	0.88
k	0.85	0.57	0.03	0.89	0.65	2.00	1.25	4.12	0.32	1.21
RMS^b	0.00	0.20	0.46	1.09	1.01	0.94	0.79	3.44	1.30	0.31

aThe COFC energy is the highest conformer energy in the series of conformers optimized with an LLM and selected as corresponding to more populated conformers in the energy range of 0–1.00 kcal/mol found by optimization with the cascade optimization method RefM2. The COFC energy determines the conformer energy range that potentially involves all the set of significant LLM conformers to be subjected to optimization at higher levels of theory.
^bThe RMS values indicate deviations of LLM energies from the energies calculated by using the RefM2.