Where
is a force constant that describes the strength of the actual type of bond and is the
equilibrium length of the bond. Both equilibrium length of the bond and force constant are
specific for each pair of bound atoms, i.e. depend on the chemical type of the atom constituents.
Values of force constant are often evaluated from experimental data such as infrared stretching
frequencies or from quantum mechanical calculations. Values of bond length can be inferred
from high resolution crystal structures or microwave spectroscopy data.