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 dont 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??