The molecules consist of balls on springs

Hooke's law

E = 1/2 K (r - r0)2

at K = 2, r0 = 1

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

Result:

r0 = 0.9215 Å