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tokens: from (numerical) frequency runs. In that case, reorganization energies (e.g. ) computed from single-point calculations were utilized to get the adiabatic values. APPENDIX A2: ADDITIONAL DETAILS ON THE CALCULATION OF ROVIBRATIONAL PROPERTIES Deceptive as it may sound for a theoretical chemist, like in the case of other carbon chains (e.g. Bâldea 2020b ), the DFT approach appears to perform better in quantifying rovibrational spectra for MgC 4 H species than much more computationally demanding ab initio methods: For MgC 4 H 0 neutral, our very tight optimization at B3LYP/aug-cc-pVTZ level yielded MHz. The relative deviation from the experimental value MHz (Cernicharo et al. 2019 ) is per cent, which is significantly better than both 0.3 per cent at CCSD(T)-F12/cc-pCVTZ-F12 (Cernicharo et al. 2019 ) and 1.7 per cent at CCSD(T)/aug-cc-pVTZ (Cernicharo et al. 2019 ). Likewise, our value D DFT = 0.0641 kHz obtained at the B3LYP/aug-cc-pVTZ level is only 13 per cent smaller than the experimental value D exp = 0.074 ± 0.002 kHz (Cernicharo et al. 2019 ). This is better than 17 per cent, the deviation of CCSD(T)-value (Cernicharo et al. 2019 , and Table A2 ), which, in turn, is somewhat better than 18 per cent, which is the deviation of the CCSD(T)-F12-based value (Cernicharo et al. 2019 ). Noteworthily, the best estimate D Kratzer = 0.0741 kHz (cf. Table 4 and A3 ) is that based on the Kratzer formula (equation 1 ) used in conjunction with the B3LYP/aug-cc-pVTZ B and ω MgC . This Kratzer-based value cannot be distinguished from the experimental value D exp = 0.074 ± 0.002 kHz (Cernicharo et al. 2019 ). Table A3. Calculated centrifugal distortion constants D (kHz) presented along with estimates deduced via Kratzer approximation (equation 1 ) using harmonic frequencies ω MgC of the MgC stretching mode. a CCSD(T)/cc-pVTZ. b CCSD(T)/aug-cc-pVTZ. c B3LYP/6-311++G(3DF, 3PD). d B3LYP/aug-cc-pVTZ. e CCSD(T)/cc-pCVTZ. Table A4. Adiabatic and vertical values of electron attachment (EA) and ionization (IP) energies computed for MgC 4 H linear chains at several levels of theory. Their most reliable estimates are depicted in italics and were used to deduce the chemical hardness η, chemical softness S , chemical potential μ, Mulliken electronegativity χ, and electrophilicity index ω. Energy unit is eV. a Δ-CCSD(T)/cc-pVTZ. b EOM-CCSD/cc-pVTZ. c Δ-CCSD(T)/cc-pCVTZ. d EOM-CCSD/cc-pCVTZ. e Δ-CCSD(T)/aug-cc-pVTZ. f .EOM-CCSD/aug-cc-pVTZ. g Δ-B3LYP/6-311++G(3DF, 3PD). h CBS-QB3. For sure, neither a DFT/B3LYP/aug-cc-pVTZ calculation like that found performing the best above nor whatever other much more sophisticated theoretical approach currently available is precise enough to enable detection in space. In view of this state-of-the-art of theory, aiming at guiding subsequent laboratory measurements and interpretation of not yet available rovibrational spectra (as the case of ), it is current practice in astrophysics (e.g. Pauzat et al. 1991 ) to estimate rotational