Stretch-Bend CouplingΒΆ
The functional forms for bond stretching, angle bending, and stretch-bend coupling are those of the MM3 force field [2]:
\[U = (k_1\Delta b_1 + k_2\Delta b_2)\Delta\theta.\]
Even though force constants k1 and k2 are implemented as two independent variables in Tinker, they were treated as the same variable in the original literature.