Nearly all DFT calculations for solids are made assuming the Born-Opperheimer approximation is valid. Let us examine this approximation:
The Born-Oppenheimer approximation is — as the name tells — an approximation. When you look at its derivation, can you imagine situations where the Born-Oppenheimer approximation is not valid? Share those with us in the forum:
Feel free to comment on the answers or suggestions by your fellow students. Are there some you did not think about? Are there others you feel must surely be wrong? Please discuss — respectfully.