What is the explanation for this discrepancy?

[JohanL]

When I calculate the Bulk modulus for a metall with the free electron model
I get a value that is twice the experimental value.

I find this strange.
With the free electron model i don't get the contribution from the ion cores and the bound electrons, right?
Do these have a negavite contribution to the bulk modulus.
If so, why?

The magnetic suceptibility has a positive contribution from the free electron paramagnetism and a negative from the diamagnetism. Where the diamagnetism comes from the ion cores and the bound electrons. This i can understand...but how can the bulk modulus have a negative contribution??

 

[Pieter Kuiper]

I never worried about this difference before. To me, it is surprising that this estimate of the compressibility is so good. After all, in equilibrium the real metal does not exert any outward pressure at all, because the positive ions keep the electrons inside.