Multiconfigurational second-order perturbation theory

Perturbative methods (CASPT2 [17], NEVPT2 [18]) add the dynamical correlation in an effective way, using multiconfigurational second-order perturbation theory on the CASSCF input states. These methods have proved to be suitable for studying problems in spectroscopy, photochemistry, and so on [19, 20]. 

Multiconfigurational Second-Order Perturbation Theory, CASPT2... 

Reaction field theory with a spherical cavity, as proposed by Karlstrom [77, 78], has been applied to the calculation of the ECD spectrum of a rigid cyclic diamide, diazabicyclo[2,2,2]octane-3,6-dione, in an aqueous environment [79], In this case, the complete active space self-consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) methods were used. The qualitative shape of the solution-phase spectrum was reproduced by these reaction field calculations, although this was also approximately achieved by calculations on an isolated molecule. 

K. Andersson and B. O. Roos, Multiconfigurational second-order perturbation theory, in D. R. Yarkony (ed.), Modem Electronic Structure Theory, Vol. I, World Scientific, Singapore, 1995, pp 55-109. 

This chapter will discuss that Complete Active Space (CAS) SCF method [1, 2] and multiconfigurational second order perturbation theory, CASPT2 [3, 4], The CASSCF method was introduced almost thirty years ago. The aim was to be able to deal with electronic structures that could not be described even qualitatively using a single electronic configuration. 

MULTICONFIGURATIONAL SECOND ORDER PERTURBATION THEORY — CASPT2... 

Andersson, K. Roos, B. O. Multiconfigurational second-order perturbation theory a test of geometries and binding enagies, Int. J. Quantum Chem. 1993,45, 591 -607. 

Dong, H. Chen, B.-Z. Huang, M.-B. Lindh, R. The Bergman cyclizations of the enediyne and its N-substituted analogs using multiconfigurational second-order perturbation theory, J. Comput. Chem. 2012, 33, 537-549. 

Roos BO, Verzazov V, Conradie J, Taylor PR, Ghosh A (2008) Not innocent verdict from Ab initio multiconfigurational second-order perturbation theory on the electronic structure of chloroiron corrole. J Phys Chem B 112 14099-14102... 

Correlation Restricted Active Space Self-Consistent Field (RASSCF) and Multiconfigurational Second-Order Perturbation Theory (CASPT2)... 

Applications of the complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) in electronic spectroscopy are reviewed. The CASSCF/CASPT2 method was developed five to seven years ago and the first applications in spectroscopy were performed in 1991. Since then, about 100 molecular systems have been studied. Most of the applications have been to organic molecules and to transition metal compounds. The overall accuracy of the approach is better than 0.3 eV for excitation energies except in a few cases, where the CASSCF reference function does not characterize the electronic state with sufficient accuracy. 

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