Double configuration

In this work we are concerned with the dressing method known as Size-Consistent Self-consistent Singles and Doubles Configuration Interaction, usually abbrebiated to (SC) SDCI [8]. This method can be considered as a generalization of the CEPA-3 formalism, as has been shown elsewhere [8,35]. It provides size-extensive roots and... 

Figure 1. The shape of the potential curve for nitrogen in a correlation-consistent polarized double-zeta basis set is presented for the variational 2-RDM method as well as (a) single-reference coupled cluster, (b) multireference second-order perturbation theory (MRPT) and single-double configuration interaction (MRCl), and full configuration interaction (FCl) wavefunction methods. The symbol 2-RDM indicates that the potential curve was shifted by the difference between the 2-RDM and CCSD(T) energies at equilibrium.

Another class of methods uses more than one Slater determinant as the reference wave function. The methods used to describe electron correlation within these calculations are similar in some ways to the methods listed above. These methods include multiconfigurational self-consistent field (MCSCF), multireference single and double configuration interaction (MRDCI), and /V-clcctron valence state perturbation theory (NEVPT) methods.5... 

An incorrect dissociation limit is a common failure of SCF MO wavefunctions (as we already noted for H2O). Thus for H2 the SCF MO wavefunction (n ) leads to a dissociation limit which is an equal mixture of atoms and ions because there is no correlation between the two electrons (there is an equal chance of finding the two electrons on the same atom and on different atoms). The addition of a configuration (cTj ), where is the lowest energy unfilled molecular orbital, removes this error, and in the dissociation limit the wavefunction has to be an equal mixture of and Thus a wavefunction that stops at this limit is called an optimum double configuration (ODC) function. 

Fig. 11. Calculations on Fj (89) Hartrcc-Fock (HF), optimized double configuration (ODC) and...

A double-configuration (MCSCF) calculation by Wood,214 of the barrier to rotation in the ground state, improves the computed barrier for both CC stretch and CH2 twisting agreement with experiment was excellent. Ahlrichs et al. have reported the results of IEPA-PNO and CEPA calculations on C2H4 and have analysed the correlation energy contributions.215... 

The above calculations on the ground state have been supplemented by calculations on a variety of excited states of H2. The n states have been very thoroughly investigated by McLean and co-workers.64-66 These workers used double-configuration wavefunctions of the type (3), where = (.ognu), [Pg.89]

Finally, we should mention some approximate calculations on H2. Jug77 has developed a semi-empirical version of the multiconfiguration SCF (MCSCF) method, using CNDO- and INDO-type approximations, and has reported the results of a double-configuration approach to Ha. It was shown that the eigenvalues of the EHF operator have physically interpretable characteristics and follow dissociation properly. Further results of this method should be very interesting. 

Neglect of Diatomic Differential Overlap Natural Spin Orbital Optimized Double Configuration Optimized Valence Configuration Potential Energy... 

Coupled-cluster singles and doubles Configuration-interaction Configuration-interaction singles... 

For the particular case of H2, very accurate calculations including correlation effects have been carried out for the potential curves of ground and some excited states of H2 by Kolos and Wolniewicz [42-44],and by Davidson [45-47], Extended Hartree-Fock calculations for the ground state [48] and double configuration SCF calculations for some excited states of H2 [49] have been carried out by Wahl and co-workers. For Hj a very rigorous calculation has been carried out by Bates and co-workers [50]. 

MP (n = 2-4) n-th order of Mpller-Plesset correlation energy correction MRSDCI multireference single - - double configuration interaction... 

The second approach is to truncate the expansion at some level of excitation. By Brillouin s theorem, the single excited configurations will not mix with the HF reference. By the Condon-Slater rules, this leaves the doubles configurations as the most important for including in the Cl expansion. Thus, the smallest reasonable truncated Cl wavefunction includes the reference and all doubles configurations (CID) ... 

The most widely employed Cl method includes both the singles and doubles configurations (CISD) ... 

B3LYP Three-parameter hybrid density functional method the MCPF method is an extension of the singles and doubles configuration interaction approach D95- " - is a Dunning double-zeta plus polarization and diffuse functions quality basis set D95 is the D95- " - basis set in which basis set functions and the polarization functions on the hydrogen atoms have not been included. 

The simplest type of MC-SCF wavefunction is one composed of just two configurations. It is often possible to choose the two configurations in such a way as to ensure that the dissociation products are correctly described. Das and Wahl first introduced this method119 arguing that it corrected the most important deficiency of the Hartree-Fock method. They called it the optimized double configuration (ODC) method. It has recently been used to study the system F + Li2 20 and was also used as a starting point in the Cl calculations on KrF2.33... 

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