Doubles, doubly excited configurations

Doubles, doubly excited configurations, 100 Dniiglas-Krnll transformation 1S ... 

Doubles, doubly excited configurations, 100 Douglas-Kroll transformation, 215 DREIDING force field, 40 Dummy atom, 37, 417 Dunning-Huzinaga basis sets, 160 Dynamic properties, 235 Dynamical electron correlation, 118... 

As we already know from the foregoing sections, the dominant part of the correlation energy is due to doubly excited configurations. It is therefore natural that attempts were made to develop such a procedure, which would pick up the contributions of doubles from the higher... 

In the case of calculations of electronic contributions to y the problem of inclusion of double excitations is much more pronounced than in the case of computations of 3. This can be illustrated as follows. Let the electronic states appearing in Eqs. (5) and (9) be classified as pure singly or doubly excited states. This is plausible under assumption that no mixing of singly and doubly excited configurations with the ground states occurs without the perturbation. According to the rules of evaluation... 

Double spin polarisation (DSP) covers the doubly excited configurations of two types ... 

The comparison of the calculated spectra of the free ions and the ones in the crystal is not straightforward. Indeed, in the crystal, the presence of the first coordination shell increases the number of electrons and basis functions in the calculations, resulting in a blow-up of the Cl expansion, mainly due to the generated doubly-excited configurations. One should bare in mind that this increase is about six time as fast in double group symmetries as in the non-relativistic symmetry. In a non effective Hamiltonian method, the only way to keep the size of the DGCI matrix to an affordable size of few million configurations, is to cut down the number of correlated electrons. This may essentially deteriorate the quality of electron correlation as the contributions of the spin-orbit interaction... 

A less intense state at 6.16 eV with 16% double excitations in the wavefunction, and two higher and intense states, also with important contributions from doubly excited configurations, complete the computed valence spectrum. The Rydberg states series are not going to be described here, but the beginning of two series were calculated at 4.85 eV (5a" 3s state) and 5.33 eV (4a"—>3s state). 

Even for DCI it is not, in general, practical to include all possible double excitations from o)> if is desirable to devise procedures for selecting the most important configurations in advance. A simple way of investigating the importance of different doubly excited configurations is to examine a perturbation treatment of this Cl eigenvalue problem. If we solve Eq. (4.28b) fore,... 

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