E = 1/2 K (r - r_{0})^{2}

The exact value of the force constant **K** is not very critical, while the equilibrium distance **r _{0}** 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

**Result:**

**r _{0} = 0.9215 Å**

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