Taking the simple case where a diatomic molecule may be considered as a simple harmonic oscillator i.e. an oscillator in which the resorting force(F) is directly proportional to the displacement , in accordance with Hook’s we have
                                                       F = -kx
Where x is the displacement and is equal to the distance to which the atoms have been stretched (R) minus equilibrium distance between the atoms (Re) i.e. x=R-Re k in equation is called force constant. Thus if x=1 cm, k=-F . Hence force constant may be defined as the restoring force per unit displacement (or per cm) of a harmonic oscillator. It is found to be related to the equilibrium vibration frequencyaccording to equation
                                                
Where is the reduced mass of the system.
             To calculate k, equation can be rewritten as follows:
                                                 
                                                
Where m₁ and m₂ are the masses of the oscillating atoms.

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