The sign of a torsional/dihedral angle among atoms i-j-k-l is positive (negative) if the vector along k-l lies to the right (left) of the plane formed by i-j-k when the plane is viewed along the j-k vector.
The sign of a torsional/dihedral angle among atoms i-j-k-l is positive (negative) if the vector along k-l lies to the right (left) of the plane formed by i-j-k when the plane is viewed along the j-k vector.