Can any one gives the idea that how to calculate the force constant (\k, as f=-kx), which must be different than the force constant obtained from the fundamental stretching expression i.e.

V_bar = (1/2*pi*c)(sqrt(k/m_reduced))

here,

v_bar  =  wave number

pi=3.14

c= velocity of light

k= force constant

m_reduced= reduced mass.