Geometrical parameters of the formic acid dimer minimum structure optimized at the reference ab initio level of theory and determined for different PESs. Bond lengths are provided in Ångströms and angles in degrees. HDNNP3 corresponds to the final FAD-HDNNP for spectroscopic use.

Parameter	Ab initio		QB16a	HDNNP1	HDNNP2	HDNNP3
	Default	Tight
r(O–H)	0.9934	0.9932	0.9927	0.9945	0.9925	0.9936
r(C–H)	1.0929	1.0930	1.0929	1.0927	1.0937	1.0927
r(C–O)	1.3113	1.3114	1.3116	1.3104	1.3121	1.3112
r(O⋯O)	2.6748	2.6758	2.6778	2.6729	2.6791	2.6709
r(C O)	1.2177	1.2176	1.2174	1.2192	1.2172	1.2178
∠O C–O	126.14	126.14	126.15	126.26	126.13	126.13
∠O C–H	122.02	122.03	122.05	122.04	122.15	121.99
∠C–O–H	109.77	109.76	109.73	109.42	109.93	109.79
∠O–H⋯O	178.93	178.93	178.95	179.51	179.01	178.86

^aQu and Bowman.⁵⁵