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Let us suppose we have a diatomic molecule, with Re as equilibrium
distance between them. Now most of authors including Herzberg's
"Spectra of Diatomic Molecules" write that we give a *displacement* R
to the molecule, and thus write the Hooke's law as:
Force = - k (R - Re) = negative ---------(1)
This expression appears true for stretching because R > Re and tells
us that stretching displacement was force are in opposite directions.
But if we, compress the diatomic molecule, and while using the same
expression and knowing that R<Re, Hooke's law will finally give a
positive sign:
Force = -k(R - Re) = positive -----------(2)
implying that force and displacement are in the same direction.