The density matrix is necessary in order to calculate the Fock matrix, but, in
turn, the Fock matrix is necessary in order to calculate the density matrix. To
break this impasse, a guessed density matrix is used. The guess is very crude:
all off-diagonal matrix elements are set to zero, and all on-diagonal terms on
any atom are set equal to the core charge of that atom divided by the number of
atomic orbitals. Our starting guess for H6 consists of a unit matrix.
Each iteration of the SCF calculation consists of assembling a Fock matrix from
the one-electron matrix, the two-electron integrals, and the density matrix,
diagonalizing it to obtain the eigenvectors, and finally reassembling the
density matrix. At some point the change in density matrix drops below a preset
limit. When this happens we say that the field is self-consistent. We will now
carry out these steps for the H6 system.