- It is well-known, that perturbation theory yields reliable results only, if the perturbation is small. This is also valid for MP2, which means, that MP2 improves HF results only, if HF already provides a fairly good solution to the problem. If HF fails, e.g. in case of partially filled d-shells, MP2 usually will also fail and should not be used in this case.
- MP2 results are known to converge very slowly with increasing basis sets, in particular slowly with increasing l-quantum number of the basis set expansion. Thus for reliable results the use of TZVPP basis sets (or higher) is recommended. When using SVP basis sets a qualitative trend can be expected at the most. Basis sets much larger than TZVPP usually do not significantly improve geometries of bonded systems, but still can improve the energetic description. For non–bonded systems larger basis sets (especially, with more diffuse functions) are needed.
- It is recommended to exclude all non-valence orbitals from MP2 calculations, as neither the TURBOMOLE standard basis sets SVP, TZVPP, and QZVPP nor the cc-pVXZ basis set families (with X=D,T,Q,5,6) are designed for correlation treatment of inner shells (for this purpose polarisation functions for the inner shells are needed). The default selection for frozen core orbitals in Define (orbitals below -3 a.u. are frozen) provides a reasonable guess. If core orbitals are included in the correlation treatment, it is recommended to use basis sets with additional tight correlation functions as e.g. the cc-pwCVXZ and cc-pCVXZ basis set families.
- RI-MP2: We strongly recommend the use of auxiliary basis sets optimized for the corresponding (MO) basis sets.

Fast RI-MP2 calculations with the ricc2 program:
As pointed out above, the ricc2 program includes (almost) all functionalities of the rimp2
program. Because of slightly refined batching algorithms, screening and symmetry
treatment the ricc2 program is usually somewhat faster than rimp2. This is in particular
the case when the molecular point group is D_{2h} or a subgroups thereof and a significant
number of atoms is positioned on symmetry elements (e.g. planar molecules) and when,
because of memory restrictions, the rimp2 program needs many passes for the integral
evaluation.

All what is needed for a RI-MP2 gradient calculation with the ricc2 program is a $ricc2
data group with the entry geoopt model=mp2. If you want only the RI-MP2 energy for a
single point use as option just mp2. To activate in MP2 energy calculations the evaluation
of the D_{1} diagnostic (for details see Sec. 10.1). use instead mp2 d1diag. (Note that the
calculation of the D_{1} diagnostic increases the costs compared to a MP2 energy evaluation
by about a factor of three.)

- 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).

Further output contains indications of the suitability of the (HF+MP2) treatment.

- 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 [108, 109] 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.