  Methyl thionitrite CH$_3$SNO is an important model of S-nitrosated cysteine
aminoacid residue (CysNO), a ubiquitous biological S-nitrosothiol (RSNO)
involved in numerous physiological processes. Here, we report accurate
structure and properties of CH$_3$SNO using accurate ab initio
Feller-Peterson-Dixon (FPD) approach. The FPD scheme included CCSD(T)-F12/CBS
extrapolated values, as well as corrections for the quadruple coupled cluster
excitations $\Delta$(Q), core-valence$\Delta$CV and scalar-relativistic
$\Delta$SR effects. The FPD scheme for the energetic parameters also included
harmonic zero-point vibrational energy (ZPE) corrected for anharmonicity. The
S-N bond length in cis-CH$_3$SNO is calculated as 1.814 {\AA}, and its
dissociation energy $D_0=32.4$ kcal/mol in the gas phase. The trans-CH$_3$SNO
conformation is 1.2 kcal/mol less stable ($\Delta E_0$) compared to
cis-CH$_3$SNO, with a sizeable cis-trans isomerization barrier $\Delta E_0^\ne
= 12.7$ kcal/mol. The paradox of the unusually long and weak S-N bond, and
hindered rotation along the S-N bond, was rationalized via the detailed
analysis of the underlying electronic structure of the -SNO group using Natural
Resonance Theory (NRT). After the benchmarking of the density functional theory
(DFT) methods against the FPD reference, we recommend mPW2PLYP and mPW2PLYPD
double hybrid functionals for calculation of the geometric properties,
vibrational frequencies and isomerization barriers of S-nitrosothiols, and PBE0
(PBE0-GD3) hybrid functional for the S-N BDEs. The abovementioned DFT methods
are capable of capturing the change in electronic structure and properties of
the -SNO fragment, when the CH$_3$SNO molecule is exposed to the influence of
physiologically feasible external electric field $F_Z$.
