Complete active space perturbation theory

Becke s three parameters Lee, Yang, Parr functional Complete Active Space Perturbation Theory 2nd Order Complete Active Space SCF Coupled Cluster Single Double CCSD (Triple)... 

The results are summarized in Tables 20.3 and 20.4. The calculations with CAS-SCF and CAS-QDPT are far too large to be done. We, therefore, compare the results with available experimental results and some recent theoretical results, i.e. MR-Cl results by Hachey et al. [39], the second-order complete active space perturbation theory (CASPT2) calculations by Merchan and Roos [40], and the equation of motion coupled cluster (EOM-CC) calculations by Gwaltney and Bartlett [41]. 

The MCSCF provides a good first-order description covering the static electron correlation due to degeneracy problems. Dynamic electron correlation should be addressed with the MCSCF wave function as a reference. The multireference configuration interaction, or MRCI, generates excited determinants from all (or selected) determinants included in the MCSCF. The complete active space perturbation theory, second order (CASPT2) is a more economical approach. Both methods can be applied to compute excited states. 

A computationally less expensive alternative to coupled-cluster theory is provided by multireference perturbation theory (MRPT). The currently most widely used form of MRPT in f element chemistry is second-order Complete Active Space Perturbation Theory (CASPT2) [79,80], and we shall focus on this method in this section. 

Anderson K, Malmqvist PA, Roos BO (1992) Second-order perturbation-theory with a complete active space self-consistent field reference function. J Chem Phys 96 1218... 

It is evident that the approach described so far to derive the electronic structure of lanthanide ions, based on perturbation theory, requires a large number of parameters to be determined. While state-of-the-art ab initio calculation procedures, based on complete active space self consistent field (CASSCF) approach, are reaching an extremely high degree of accuracy [34-37], the CF approach remains widely used, especially in spectroscopic studies. However, for low point symmetry, such as those commonly observed in molecular complexes, the number of CF... 

Order Perturbation Theory with a Complete Active Space Self-Consistent Field Reference Function. 

Nickel Atom Studied With the Complete Active Space SCF Method and Second-Order Perturbation Theory. 

In ab initio methods the HER approximation is used for build-up of initial estimate for and which have to be further improved by methods of configurational interaction in the complete active space (CAS) [39], or by Mpller-Plesset perturbation theory (MPn) of order n, or by the coupled clusters [40,41] methods. In fact, any reasonable result within the ab initio QC requires at least minimal involvement of electron correlation. All the technical tricks invented to go beyond the HFR calculation scheme in terms of different forms of the trial wave function or various perturbative procedures represent in fact attempts to estimate somehow the second term of Eq. (5) - the cumulant % of the two-particle density matrix. 

CASPT2 Complete active space second-order perturbation theory... 

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