Recently I want to get insight into the 3d states of 3d transition metals and their alloys.

To avoid unnecessary interaction between the localized 3d states and other valence electrons, I want to use a state of art approach, such as GW to calculate the band structure for e.g. Cu.

However, the calculations provided llarge energy gap for FCC Cu.

This looks very unreasonable.

Please help me about this issue.

Thanks in advance.