- Most important output for
`ricc2`,`rimp2`, and`mpgrad`are of course MP2(+HF) energies (written standard output and additionally to file`energy`) and MP2(+HF) gradients (written to file`gradient`). - In case of MP2 gradient calculations the modules also calculate
the MP2 dipole moment from the MP2 density matrix (note, that in
case of
`mpgrad`frozen core orbital specification is ignored for gradient calculations and thus for MP2 dipole moments).

- As discussed above, reliable (HF+MP2) results are in line with
small MP2 corrections. The size of the MP2 correction is
characterised by the t-amplitudes, as evident from the above
equations.
`mpgrad`by default plots the five largest t-amplitudes as well as the five largest norms of t-amplitudes for fixed*i*and*j*,`rimp2`does the same upon request, if`$tplot`is added to`control`file. More or less than five t-amplitudes will be plotted for`$tplot`*n*, where*n*denotes the number of largest amplitudes to be plotted. It is up to the user to decide from these quantities, whether the (SCF+MP2) treatment is suited for the present problem or not. Unfortunately, it is not possible to define a threshold, which distinguishes a "good" and a "bad" MP2-case, since the value of individual t-amplitudes are not orbital-invariant, but depend on the orbital basis (and thereby under certain circumstances even on the orientation). Example: the largest norm of t-amplitudes for the Cu-atom (*d*^{10}*s*^{1}, "good" MP2-case) amounts to ca. 0.06, that of the Ni-atom (*d*^{8}*s*^{2}, "bad" MP2 case) is ca. 0.14. - A more descriptive criterion may be derived from the MP2 density
matrix. The eigenvalues of this matrix reflect the changes in
occupation numbers resulting from the MP2 treatment, compared to the
SCF density matrix, where occupation numbers are either one (two for
RHF) or zero. Small changes mean small corrections to HF and thus
suitability of the (HF+MP2) method for the given problem. In case of
gradient calculations
`rimp2`displays by default the largest eigenvalue of the MP2 density matrix, i.e. the largest change in occupation numbers (in %). All eigenvalues are shown, if`$mp2occ`is added to the`control`file. For main group compounds largest changes in occupation numbers of ca. 5% or less are typical, for*d*^{10}metal compounds somewhat higher values are tolerable. - A similar idea is pursued by the
*D*_{2}and*D*_{1}diagnostics [93,94] which is implemented in`ricc2`.*D*_{2}is a diagnostic for strong interactions of the HF reference state with doubly excited determinants, while*D*_{1}is a diagnostic for strong interactions with singly excited determinants.