The CC2 ground-state energy is—similarly to other coupled-cluster energies—obtained from the expression
|Ωμ1||= ⟨μ1|Ĥ + [Ĥ,T2]|HF⟩ = 0 ,||(10.5)|
|Ωμ2||= ⟨μ2|Ĥ + [F,T2]|HF⟩ = 0 ,||(10.6)|
|Ĥ = exp(-T1)H exp(T1),|
The residual of the cluster equations Ω(tai,taibj) is the so-called vector function. The recommended reference for the CC2 model is ref. , the implementation with the resolution-of-the-identity approximation, RI-CC2, was first described in ref. .
Advantages of the RI approximation: For RI-CC2 calculations, the operation count and thereby the CPU and the wall time increases—as for RI-MP2 calculations—approximately with (O2V 2Nx), where O is the number of occupied and V the number of virtual orbitals and Nx the dimension of the auxiliary basis set for the resolution of the identity. Since RI-CC2 calculations require the (iterative) solution of the cluster equations (10.5) and (10.6), they are about 10–20 times more expensive than MP2 calculations. The disk space requirements are approximately O(2V + N)Nx + Nx2 double precision words. The details of the algorithms are described in ref. , for the error introduced by the RI approximation see refs. [117,135].
Required input data: In addition to the above mentioned prerequisites ground-state energy calculations with the ricc2 module require only the data group $ricc2 (see Section 21.2.17), which defines the methods, convergence thresholds and limits for the number of iterations etc. If this data group is not set, the program will carry out a CC2 calculation. With the input
the ricc2 program will calculate the MP2 and CC2 ground-state energies, the latter converged to approximately 10-6 a.u. The solution for the single-substitution cluster amplitudes is saved in the file CCR0--1--1---0, which can be kept for a later restart.
Ground-State calculations for other methods than CC2: The MP2 equations and the energy are obtained by restricting in the CC2 equations the single-substitution amplitudes tai to zero. In this sense MP2 can be derived as a simplification of CC2. But it should be noted that CC2 energies and geometries are usually not more accurate than MP2.
For CCS and CIS the double-substitution amplitudes are excluded from the cluster expansion and the single-substitution amplitudes for the ground state wavefunction are zero for closed–shell RHF and open–shell UHF reference wavefunctions and thus energy is identical to the SCF energy.
For the Methods CIS(D), CIS(D∞) and ADC(2) the ground state is identified with the MP2 ground state to define is total energy of the excited state, which is needed for the definition of gradients and (relaxed) first-order properties which are obtained as (analytic) derivatives the total energy.
Diagnostics: Together with the MP2 and/or CC2 ground state energy the program evaluates the D1 diagnostic proposed by Janssen and Nielsen , which is defined as:
where λmax[M] is the largest eigenvalue of a positive definite matrix M. (For CC2 the D1 diagnostic will be computed automatically. For MP2 is must explictly be requested with the d1diag option in the $ricc2 data group, since for RI-MP2 the calculation of D1 will contribute significantly to the computational costs.) Large values of D1 indicate a multireference character of the ground-state introduced by strong orbital relaxation effects. In difference to the T1 and S2 diagnostics proposed earlier by Lee and coworkers, the D1 diagnostic is strictly size-intensive and can thus be used also for large systems and to compare results for molecules of different size. MP2 and CC2 results for geometries and vibrational frequencies are, in general, in excellent agreement with those of higher-order correlation methods if, respectively, D1(MP2) ≤ 0.015 and D1(CC2) ≤ 0.030 [11,118]. For D1(MP2) ≤ 0.040 and D1(CC2) ≤ 0.050 MP2 and/or CC2 usually still perform well, but results should be carefully checked. Larger values of D1 indicate that MP2 and CC2 are inadequate to describe the ground state of the system correctly!
The D2 diagnostic proposed by Nielsen and Janssen  can also be evaluated. This analysis can be triggered, whenever a response property is calculated, e.g. dipole moment, with the keyword $D2-diagnostic. Note that the calculation of D2 requires an additional O(N5) step! D2(MP2/CC2) ≤ 0.15 are in excellent agreement with those of higher-order correlation methods, for D2(MP2/CC2) ≥ 0.18 the results should be carefully checked.