Hi, The following textbook Heisenberg's Quantum Mechanics shows an example of calculating Berry's curvature (top page on pg 518). It led to a following equation Vm= (- 1/B2 ) * i *∑ ( <m,B|S|n,B> ∧ <n,B|S|m,B> ) / A2 ................ the textbook claims that we add the term m = n since <m|S|m> ∧ <m|S|m> = 0 then the above equation simplifies to Vm= (- 1/B2 ) * i * ∑( <m,B|S ∧ S|n,B> ) / A2................. The symbol ∧ stands for 'and' in logic or cross product. My question is how the author derived that claim and how it led to that equation  from equation  ? My reasoning is that |n,B> 'and' <n,B| are both true therefore ( |n,B> 'and' <n,B|) = 1 which is equal to 1 'and' 1 however I do not believe that my reasoning is a valid one. What is the alternative to this? Thanks.