====== The molecules consist of balls on springs ======

{{:hf.png?300}}

[[http://en.wikipedia.org/wiki/Hooke%27s_law|Hooke's law]]

 E = 1/2 K (r - r<sub>0</sub>)<sup>2</sup>

{{:gook.png?250}} at K = 2, r<sub>0</sub> = 1

The exact value of the force constant **K** is not very critical, while the equilibrium distance **r<sub>0</sub>** 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 [[http://www.nwchem-sw.org|NWChem 6.1]]

<code>
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
</code>

**Result:**

**r<sub>0</sub> = 0.9215 Å**