Table 1: Equilibrium geometry (in $\AA$ ) dipole moment ( $\mu$ , in Debyes), rotational constants ( $B_{\rm e}$ , $D_{\rm e}$ and B₀, in MHz), harmonic wavenumbers ( $\omega _{i}$ , in cm-1), and intensities (I, in Debyes²/ $\AA^{2}$ amu) of l-SiCCC.
RCCSD(T) RCCSD(T) UCCSD(T) CASSCF CASSCF CASSCF MCSCF CCSD(T) MP2 CISD $m\omega^g$

cc-pVDZ^a cc-pVTZ^b cc-pVTZ^b cc-pVDZ^a cc-pVTZ^b cc-pVQZ^b 6-31G(d)^c cc-pVQZ^d 6-31G(d)^e DZP^f

R₁ (Si-CCC str) 1.7662 1.7447 1.7427 1.7531 1.7410 1.7197 1.74 1.7249 1.732 1.722

R₂ (SiC-CC str) 1.3299 1.2959 1.2954 1.2966 1.2841 1.2956 1.29 1.2899 1.297 1.298

R₃ (SiCC-C str) 1.3120 1.3108 1.3133 1.3111 1.3003 1.2987 1.31 1.3062 1.3111 1.3070

$\mu$ 4.0293 4.4064

$B_{\rm e}$ 2636.4 2705.49 2706.86 2691.68 2736.88 2753.16 2747.7085

B₀ 2637.8 2690.45

$D_{\rm e} \times 10^{-6}$ 221.529 218.844 255.23

$\omega_1 (\sigma)$ (C-C-C stretching) 2016.2 2040 1971 2070.0 2060 2005 (I=152) 2055 2003

$\omega_2 (\sigma)$ (Si-C stretching) 1368.8 1391 1318 1357.1 1365 1366 (I=11) 1342 1332

$\omega_3 (\sigma)$ (C-C-C stretching) 593.8 612 606 611.3 621 639 (I=4) 613 629

$\omega_4 (\pi)$ (trans-bending) 379.8 384 381 443.1 455 463 (I=9) 382 380

$\omega_5 (\pi)$ (cis-bending) 147.2 147 147 166.9 175 166 (I=3) 150 151

**Table 1:** Equilibrium geometry (in $\AA$ ) dipole moment ( $\mu$ , in Debyes), rotational constants ( $B_{\rm e}$ , $D_{\rm e}$ and B₀, in MHz), harmonic wavenumbers ( $\omega _{i}$ , in cm-1), and intensities (I, in Debyes²/ $\AA^{2}$ amu) of l-SiCCC.
		RCCSD(T)	RCCSD(T)	UCCSD(T)	CASSCF	CASSCF	CASSCF	MCSCF	CCSD(T)	MP2	CISD	$m\omega^g$
		cc-pVDZ^a	cc-pVTZ^b	cc-pVTZ^b	cc-pVDZ^a	cc-pVTZ^b	cc-pVQZ^b	6-31G(d)^c	cc-pVQZ^d	6-31G(d)^e	DZP^f
R₁	(Si-CCC str)	1.7662	1.7447	1.7427	1.7531	1.7410	1.7197	1.74	1.7249	1.732	1.722
R₂	(SiC-CC str)	1.3299	1.2959	1.2954	1.2966	1.2841	1.2956	1.29	1.2899	1.297	1.298
R₃	(SiCC-C str)	1.3120	1.3108	1.3133	1.3111	1.3003	1.2987	1.31	1.3062	1.3111	1.3070
$\mu$						4.0293	4.4064
$B_{\rm e}$		2636.4	2705.49	2706.86	2691.68	2736.88	2753.16					2747.7085
B₀		2637.8			2690.45
$D_{\rm e} \times 10^{-6}$		221.529			218.844							255.23
$\omega_1 (\sigma)$	(C-C-C stretching)	2016.2	2040	1971	2070.0	2060	2005 (I=152)	2055		2003
$\omega_2 (\sigma)$	(Si-C stretching)	1368.8	1391	1318	1357.1	1365	1366 (I=11)	1342		1332
$\omega_3 (\sigma)$	(C-C-C stretching)	593.8	612	606	611.3	621	639 (I=4)	613		629
$\omega_4 (\pi)$	(trans-bending)	379.8	384	381	443.1	455	463 (I=9)	382		380
$\omega_5 (\pi)$	(cis-bending)	147.2	147	147	166.9	175	166 (I=3)	150		151

^a This work. Values derived from our 6D PES; ^b this work. Values obtained using standard approaches implemented in MOLPRO. Refs. (Eckert et al. 1997) (Rauhut et al. 1999); ^c Ref. (Rintelman et al. 2001); ^d Ref. (Sattelmeyer et al. 2002); ^e Ref. (Gomei et al. 1997); ^f Ref. (Alberts et al. 1990); ^g Ref. (McCarthy et al. 2000).

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