Comparison of the harmonic frequencies ω_i (in cm⁻¹) for the three HDNNPs and the respective reference ab initio results obtained at the fc-CCSD(T)-F12a/haTZ level of theory. The latter are computed using either the standard convergence criteria (termed default) or stricter convergence criteria (termed tight) with respect to the electronic energy and geometry optimization as well as smaller step sizes in the numerical gradient and Hessian (cf. Section 3).

Mode	Sym.	HDNNP1	HDNNP2	FAD-HDNNP	Ab initio
				HDNNP3	Default	Tight
ω 1	A g	3218	3212	3209	3203	3207
ω 2	A g	3060	3077	3103	3105	3103
ω 3	A g	1708	1723	1722	1717	1718
ω 4	A g	1443	1485	1486	1484	1482
ω 5	A g	1376	1417	1410	1413	1411
ω 6	A g	1254	1249	1257	1257	1255
ω 7	A g	689	681	688	688	686
ω 8	A g	216	215	214	211	210
ω 9	A g	164	173	167	171	170
ω 10	B g	1073	1083	1083	1085	1083
ω 11	B g	980	960	957	960	955
ω 12	B g	257	241	257	258	249
ω 13	A u	1128	1106	1109	1102	1101
ω 14	A u	937	983	979	986	985
ω 15	A u	213	161	180	186	173
ω 16	A u	68	71	71	76	68
ω 17	B u	3324	3323	3312	3305	3309
ω 18	B u	3108	3085	3100	3101	3099
ω 19	B u	1773	1783	1784	1782	1782
ω 20	B u	1411	1459	1459	1456	1453
ω 21	B u	1358	1414	1410	1405	1407
ω 22	B u	1208	1247	1260	1260	1260
ω 23	B u	704	718	712	716	715
ω 24	B u	281	275	275	278	276