CT-Nitro group N torsion

[bannanc]

It seems smirnoff doesn't have anything for torsions with a tetravalent carbon next to a nitro group
Such as:

In parm@frosst, these for atoms are CTCTN2~O2 which get the more generic torsion X-CT-N2-X

X -CT-N2-X    6    0.00          0.0             3.         JCC,7,(1986),230

Here is the X-CT-N2-X torsion in smirff:

[*:1]-[#6X4:2]-[#7X3$(*~[#6X3,#6X2]):3]~[*:4]  1  0.000    0.0  -2.0  parm99 X_-CT-N2,N,N*-X  JCC,7,(1986),230
[*:1]-[#6X4:2]-[#7X3$(*~[#6X3,#6X2]):3]~[*:4]  1  0.000    0.0   3.0  parm99 X_-CT-N2,N,N*-X  JCC,7,(1986),230

The SMIRKS pattern for a nitro group is [#7X3$(*~[#8X1])$(*~[#8X1])] we could or it so the smirks was [*:1]-[#6X4:2]-[#7X3$(*~[#6X3,#6X2]),#7X3$(*~[#8X1])$(*~[#8X1]):3]~[*:4] but that is getting really complicated, I vote we just add a line for the specific nitro-CT torsions

[*:1]-[#6X4:2]-[#7X3$(*~[#6X3,#6X2]):3]~[*:4]  1  0.000    0.0  -2.0  parm99 X_-CT-N2,N,N*-X  JCC,7,(1986),230
[*:1]-[#6X4:2]-[#7X3$(*~[#6X3,#6X2]):3]~[*:4]  1  0.000    0.0   3.0  parm99 X_-CT-N2,N,N*-X  JCC,7,(1986),230
[*:1]-[#6X4:2]-[#7X3$(*~[#8X1]):3]~[#8X1:4]     1 0.000    0.0    3.0 parm99 X-CT-N2-X or GAFF2 X-c3-no-X 

[bannanc]

Closed by pull request #43