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