Table A.1:

Relative stability (kcal/mol) and dipole moments (Debye) of isomers with same generic formula.

Generic	Most stable		Other
formula	isomer	B3LYPa/b	isomers	B3LYPa/b	B3LYPa	B3LYPb	CCSD(T)c
HCO+	HCO+	3.7/3.7	HOC+	2.6/2.6	37.3	36.7	37.9
HCN	HCN	2.9/3.0	HNC	3.1/3.1	14.3	13.6	14.5
C3H	cyc-C3H	3.6/3.6	C3H	2.3/2.3	-1.7	-1.0	1.2
C3H2	cyc-C3H2	3.3/3.5	l-C3H2	4.2/4.4	11.1	11.8	13.7
HNCO	HNCO	2.2/2.1	HCNO	3.1/3.2	67.8	68.0	69.0
C2H3N	CH3CN	3.9/4.0	CH3NC	3.9/3.9	24.1	23.5	24.3
			CH2CNH	1.5/1.4	23.5	22.1	27.1
C2H4O	CH3CHO	2.7/2.8	CH2CHOH	1.0/0.9	11.6	10.5	10.3
			cyc-C2H4O	2.0/1.9	30.5	29.2	27.8
C2H6O	CH3CH2OH	1.6/1.6	CH3OCH3	1.3/1.2	9.9	10.4	12.0
HC3N	HC3N	3.8/3.9	HC2NC	2.9/2.9	25.5	25.0	26.8
			HNC3	5.4/5/5	46.0	44.8	51.4
C3H2O	HC2CHO	2.8/2.9	cyc-H2C3O	4.0/4.2	4.8	5.2	6.8
C2H4O2	CH3COOH	1.6/1.7	HCOOCH3	1.8/1.9	16.0	16.1	17.1
			HOCH2CHO	2.4/2.4	28.5	27.2	27.2
C4H3N	CH3-CC-CN	5.2/5.3	CH2=C=CHCN	4.3/4.4	3.0	2.6	3.8
MgNC	MgNC	5.2/5.3	MgCN	5.2/5.4	1.9	2.3	1.4
SiCN	SiCN	2.9/3.1	SiNC	2.6/2.7	1.2	1.3	1.5

Notes. The first column reports the generic formula of the molecules, ordered in increasing molecular weight. The next two columns report the most stable isomer with its dipole moment; fourth and fifth columns report the less abundant isomer(s) with the relevant dipole moment; Cols. 6 to 8 report the relative stability with respect to the most stable (Col. 2) computed using the three different methods discussed in Sect. 2. ^(a) Basis set: 6-311G(d,p); ^(b) Basis set: cc-pVQZ; ^(c) basis set: cc-pVQZ.