Hi, I'm a physics undergraduate, and I'm having trouble understanding Atomic Physics right now. At first I thought it (the question in the title) is possible, but then I think I got confused with particle in a box. So I refer to textbooks and look for answers in the internet, and confirmed that we can only solve it by using spherical coordinates. But I couldn't find the reason why we should only use spherical coordinates, but not rectangular coordinates. My only guess is it's because atoms are said to be spherical. Am I correct? Whether I'm right or not, can anyone elaborate this? Thank you in advance.