B. Venkataraman and G. K. Fraenkel, J. Chem. Phys, p.737, 1956.

H. M. Mc-connell, J. Chem. Phys, p.764, 1956.

R. Bernsohn, J. Chem. Phys, p.1066, 1956.

S. I. Weissman, J. Chem. Phys, p.890, 1956.

H. Kollmar and W. Staemmler, Theor. Chim. Acta, vol.48, p.223, 1978.

A. F. Voter, M. M. Goodgame, W. A. Goddard, and I. , Chem. Phys, vol.98, pp.7-14, 1985.

M. Abe, Chem. Rev, vol.113, pp.7011-7088, 2011.

J. A. Berson, Acc. Chem. Res, vol.30, pp.238-244, 1997.

M. Filatov and S. Shaik, J. Phys. Chem. A, vol.103, pp.8885-8889, 1999.

E. Rodríguez, M. Reguero, and R. Caballol, J. Phys. Chem. A, vol.104, pp.6253-6258, 2000.

Z. D. Pozun, X. Su, and K. D. Jordan, J. Am. Chem. Soc, vol.135, pp.13862-13869, 2013.

M. Barborini and E. Coccia, J. Chem. Theory Comput, vol.11, pp.5696-5704, 2015.

L. Veis, A. Antalík, Ö. Legeza, A. Alavi, and J. Pittner, J. Chem. Theory Comput, vol.14, pp.2439-2445, 2018.

J. Malrieu, R. Caballol, C. J. Calzado, C. De-graaf, and N. Guihéry, Chem. Rev, vol.114, p.429, 2014.

J. A. Pople and R. K. Nesbet, J. Chem. Phys, vol.22, pp.571-572, 1954.

G. Berthier, J. Chim. Phys, vol.51, pp.363-371, 1954.

A. P. Ginsberg, J. Am. Chem. Soc, vol.102, p.111, 1980.

L. Noodleman, J. Chem. Phys, vol.74, p.5737, 1981.

K. Yamaguchi, H. Fukui, and T. Fueno, Chem. Lett, pp.625-628, 1986.

K. Yamaguchi, Y. Takahara, T. Fueno, and K. N. Houk, Theor. Chim. Acta, vol.73, pp.337-364, 1988.

S. Yamanaka, M. Okumura, M. Nakano, and K. Yamaguchi, J. Mol. Struct. THEOCHEM, vol.310, pp.205-218, 1994.

N. Ferré, N. Guihéry, and J. Malrieu, Phys. Chem. Chem. Phys, vol.17, pp.14375-14382, 2015.

T. Takahashi and G. Scuseria, J. Chem. Phys, p.64101, 2011.

C. A. Jiménez-hoyos, T. M. Henderson, T. Tsuchimochi, and G. E. Scuseria, J. Chem. Phys, p.164109, 2012.

P. Rivero, C. A. Jiménez-hoyos, and G. E. Scuseria, J. Phys. Chem A, vol.117, pp.8973-8080, 2013.

, 27 Full geometry optimizations are carried out up to energy gradients lower than10 ?5 a.u. Cartesian coordinates for all optimized geometries are available upon request

M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb et al., , 2013.

, At the present level of description, and neglecting ZPE corrections, the CC dissociation energy in ethane is calculated at 93.3 kcal/mol, The spin-decontamination correction would therefore lower this value to 87.9 kcal/mol. The agreement with experimental bond dissociation enthalpy in ethane at 0 K (87.9 kcal/mol)

. Ruscic, J. Phys. Chem A, vol.119, pp.7810-7837, 2015.

G. Trinquier and J. Malrieu, J. Phys. Chem. A, vol.116, pp.10864-10869, 2012.

E. Coulaud, N. Guihéry, J. Malrieu, D. Hagebaum-reignier, D. Siri et al., J. Chem. Phys, vol.137, p.114106, 2012.

E. Coulaud, J. P. Malrieu, N. Guihéry, and N. Ferré, J. Chem. Theory Comput, issue.9, p.3429, 2013.

C. J. Calzado, J. Cabrero, R. Caballol, and J. Malrieu, The demonstration of the formula is given on page 2735. There is a misprint in equation 46, where a minus sign, J. Chem. Phys, vol.116, pp.2728-2747, 2002.

A. A. Ovchinnikov, Theor. Chim. Acta, vol.47, pp.297-304, 1978.

R. J. Buenker and S. Peyerimhoff, Chem. Phys, vol.9, pp.75-81, 1975.

, For recent theoretical works on ethylene torsional barrier, see: E. San-Fabian and F. Moscardo, J. Comput. Chem, vol.35, pp.1356-1363, 2014.

S. S. Ray, U. S. Mahapatra, R. K. Chaudhuri, and S. Chattopadhyay, Comput. Theor. Chem, vol.1120, pp.56-78, 2017.

G. Trinquier, N. Suaud, N. Guihéry, and J. Malrieu, Chem. Phys. Chem, vol.12, pp.3020-3036, 2011.