E = 1/2 K (r - r0)2
The exact value of the force constant K is not very critical, while the equilibrium distance r0 is desirable to determine accurately. This can be done with the help of quantum chemical calculations in the basis set of average size, such as 6-311+G(2d,p). It is important to take into account the electron correlation, for example by means of MP2.
Example of calculation of the equilibrium distance for a hydrogen fluoride (HF) molecule. Entry to the program NWChem 6.1
charge 0 geometry F 0.71273369962679 0.06663643064244 0.00000000000000 H 1.19708630037321 0.86172356935756 0.00000000000000 end basis cartesian * library 6-311+G(2d,p) end driver clear DEFAULT eprec 1e-06 maxiter 2000 end mp2 tight freeze atomic end task MP2 optimize
r0 = 0.9215 Å