Correlation energy relativistic, second order

In preceding sections we have discussed several different relativistic methods four-component Dirac—Fock with and without correlation energy, the second-order Douglas—Kroll method, and perturbation methods including the mass—velocity and Darwin terms. The relativistic effective core potential (RECP) method is another well-established means of accounting for certain relativistic effects in quantum chemical calculations. This method is thoroughly described elsewhere - anJ is basically not different in the relativistic and... 

The plan of this paper is as follows - In section 2, the basic experimental data required in the re-evaluation of the empirical correlation energies of the N2 CO, BF and NO molecules are collected. The essential theoretical ingredients of our re-determination are given in section 3 including new fully relativistic calculations including the frequency independent Breit interaction and electron correlation effects described by second order diagrammatic perturbation theory for the Be-like ions B", C, O" ... 

In order to establish the plausibility of the argument that the calculation of relativistic corrections to the second-order correlation energy in Be-like ions may be used to estimate reliably the corresponding quantity in many-electron molecules, we have included a calculation, in Table 8, of the many-body corrections to Ne. The relativistic correction to the correlation derived from these calculations is 1.257 x 10 hartree, which should be compared with our previous calculations [38] of the same quantity in 1.277 x 10 hartree, and with the independent calculations by Drake... 

From this, we may deduce that the relativistic correction to the correlation energy is dominated by the contribution from the s electron pair, and that the total relativistic effect involving the exchange of a single transverse Breit photon is obtained to sufficient accuracy for our present purposes at second-order in many-body perturbation theory. 

The CASSCF wavefiinction is used as reference function in a second-order estimate of the remaining dynamical correlation effects. All valence electrons were correlated in this step and also the 3s and 3p shells on copper. Relativistic corrections (the Darwin and mass-velocity terms) were added to all CASPT2 energies. They were obtained at the CASSCF level using first-order perturbation theory. A level-shift (typically 0.3 Hartree) was added to the zeroth order Hamiltonian in order to remove intruder states [30]. Transition moments were conputed with the CAS state-interaction method [31] at the CASSCF level. They were... 

Figure 3. Comparison of total atomic correlation energies from several sources difference between full and HF subhamiltonians ( ) difference between total ionization energies and accurate HF calculations with relativistic corrections [15,16] (x) linear extrapolation of the latter values [20] (--) second-order M0ller-Plesset calculations [17,18]...

---