Table 1.

Best-fit data for various combinations of electrostatic variables evaluated for the CHARMM27 force field against experimental frequencies for our 23 standard dipeptides. The first data column presents the sample standard deviation (σ) of the error between best-fit prediction and experimental values. The second column reports the predicted zero-field (i.e., vacuum) frequency, and the third column the linear coefficients for the respective variables (see, e.g., Eqs. 4 and (8)). The units on the potential are E_h/e and on the field E_h/a_oe, as above. Note that for the potential fits, the coefficients are constrained to sum to zero.

	σ (cm−1)	ωo (cm−1)	Electrostatic coefficients
$E_x^{\mathrm{O}}$	2.79	1710.3	$+2707.8·E_x^{\mathrm{O}}$
$E_x^{\mathrm{C}}/E_x^{\mathrm{N}}$	4.92	1677.9	$+2557.8·E_x^{\mathrm{C}}$
			$-1099.5·E_x^N$
ΦC/ΦO	2.95	1702.6	+1136.5·ΦC
			−1136.5·ΦO
4-site Φ	2.77	1724.9	+1123.5·ΦC
			−1383.4·ΦO
			−11.819·ΦN
			+271.66·ΦH