Hi,

I have a question regarding the eigendecomposition used in the psi4 code, and the resulting order of the eigenvectors. I would like to present my understanding of the code, and would very much appreciate it, if someone could correct my mistakes.

Psi4 constructs the Overlap matrix S, the one electron Matrix H, and the remaining parts needed for the Kohn-Sham Matrix/Fock-Matrix F.

Using S, F is converted to F_prime. The eigenvalues of F_prime are my orbital energies, and the eigenvectors form my C_prime matrix, which is converted to C using S^{-1/2}. The vectors in C are sorted according to their orbital energies, such that the density matrix is easily constructed considering the first N/2 entries. Now, using the new density matrix, the algorithm is repeated until convergence. The matrices C_prime, F, F_prime, S and H are never sorted. When C is printed to file, psi4 prints the sorted C matrix. Is that correct?

All the Best

Lenoi