Multiconfigurational second-order structure

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. 

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... 

Frequency-dependent response functions can only be computed within approximate electronic structure models that allow definition of the time-dependent expectation value. Hence, frequency-dependent response functions are not defined for approximate methods that provide an energy but no wave function. Such methods include MoUer-Plesset (MP) perturbation theory, multiconfigurational second-order perturbation theory (CASPT2), and coupled cluster singles and doubles with non-iterative perturbative triples [CCSD(T)]. As we shall see later, it is possible to derive static response functions for such methods. 

Coupling of quantum mechanical molecular subsystems with larger classically treated subsystems has traditionally involved electronic structure models describing molecules embedded in a dielectric medium and this is a research area that has expanded tremendously over the last three decades [2-36]. Most of this work has involved electronic structure methods that have been based on uncorrelated electronic structure methods [2-12,15-19]. Accurate description of the electronic structure of molecular systems requires that the correlated electronic motion in the molecule is incorporated and therefore a number of correlated electronic structure methods have been developed such as the second order Moller-Plesset (MP2) [28,30,90,91], the multiconfigurational self-consistent reaction field (MCSCRF) [13,20] and the coupled-cluster self-consistent reaction field (CCSCRF) method [36]. 

The present contribution concerns an outline of the response tlieory for the multiconfigurational self-consistent field electronic structure method coupled to molecular mechanics force fields and it gives an overview of the theoretical developments presented in the work by Poulsen et al. [7, 8, 9], The multiconfigurational self-consistent field molecular mechanics (MCSCF/MM) response method has been developed to include third order molecular properties [7, 8, 9], This contribution contains a section that describes the establisment of the energy functional for the situation where a multiconfigurational self-consistent field electronic structure method is coupled to a classical molecular mechanics field. The second section provides the necessary background for forming the fundamental equations within response theory. The third and fourth sections present the linear and quadratic, respectively, response equations for the MCSCF/MM response method. The fifth 283... 

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