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286 P. LAZZERETTI ET AL.
where
is used to start the iteration (49)-(52). In the case of electric perturbation, denoting
by the coordinates of the i-th electron with charge –e,
To first order the repulsion matrix
is obtained contracting the first-order density matrix with the two-electron integrals
over the atomic basis,
The second-order density matrix
is also resolved into four components according to (46),
that is,
Accordingly, only the projections over the subspace of virtual orbitals are needed
to compute the second-order density matrix.
The iterative scheme for the second-order coefficients, consistent with (49)-(52), is
(in the case of electric perturbation there is no
