In the direct Davidson diagonalization method, the computational bo leneck is the matrix product W = H U, and only this step needs to be distributed. e calculation is divided into independent tasks where each task builds a unique piece of W containing 40 000 consecutive determinants. Communicating the result of all the tasks scales as O(N det ), independently of the number of parallel processes. On the other hand, U needs to be broadcast e ciently at the beginning of the calculation to each slave process.