Correction for torsion angles asymptotics¶

There is a discontinuity problem for small bond orders in the expression for torsion angles and conjugation contributions f(BO). These terms correspond to expressions for f₁₀ (eq. 10b) and f₁₂ (eq. 11b) in the original ReaxFF paper “ReaxFF: A Reactive Force Field for Hydrocarbons”, J. Phys. Chem. A 2001, 105, 9396-9409. The new expression for each term in f₁₀ is [1 - exp(-2 * lambda ₂₃ * BO**2)] and in f₁₂ it is sin(BO*pi/3)**4. The new expressions ensure correct asymptotic behavior for the dE/dBO for BO \(\rightarrow\) 0. Using this makes geometry optimizations more stable and improves conservation of energy during MD.

The correction can be enabled by setting the tors13 flag in the control file to 1.