Why can't they solve it on a test even though they 'understand it when reading the explanation'?

"They said they understood it in class, but they can't solve it on the test.""They are convinced when they read the ...

DFT Part 2: Why do orbitals appear in a density theory? — The Kohn–Sham method

IntroductionLast time, we introduced the Hohenberg–Kohn theorem, which is the starting point of Density Functional Theory ...