Bond Stretching¶
Bond term is an empirical function of bond deviating from the ideal bond length, i.e., \(\Delta b = b - b_0\). To support the AMOEBA force field, Tinker includes the 3rd and 4th order terms.
\[U = k\Delta b^2(1 + k_3\Delta b + k_4\Delta b^2).\]
Note
Different from Hooke’s Law, \(U=kx^2/2\), Tinker drops the coeffient 1/2.
The Morse oscillator is also implemented in Tinker:
\[U = D_e[1 - \exp(-a\Delta b)]^2.\]
Parameter a is hardwired to 2 by approximation. Following equation \(a = \sqrt{\frac{k}{2 D_e}}\) and the Tinker convention to include 1/2 in the force constant, De is k/4.