Torsional force equation in AIREBO code

In the Sturat paper the Vtors has half angle of the cos but looking at the source code of airebo.cpp line number 1035 it is given as: cw2 = (.5*(1.0-cw)). But in order to find the half angle square of the cos shouldnt there be positive sign? Or I am missing something.

The AIREBO code in LAMMPS has been validated against independent implementations multiple times, so we are quite confident that the forces it computes are correct.

I am not very familiar with the details of the implementation, perhaps @athomps has an idea.