However, the degree of correlation introduced in these methods depends on the form of the so-called exchange-correlation potential and it is difficult to estimate how much correlation is present in the results unless performing comparative ab initio calculations. As far as they are concerned, the techniques based on density functional theory (DFT) are able to incorporate some treatment of correlation through the energy functional used to solve the Kohn-Sham equations.
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For example, most of the popular semiempirical models offer the possibility to include some CI using, e.g., the Pople-Pariser-Parr formalism, as implemented in theĪMPAC series of programs. Fortunately, substantial efforts have been made in the last twenty years in order to develop correlated quantum chemical methods and there is ample choice among them today for a given problem. On the other hand, it is indispensable for most molecular properties to account for correlation effects so as to achieve quantitative, or even sometimes qualitative, predictions as the neglect of instantaneous repulsions introduces an error which may be significant. , "the choice of an appropriate molecular orbital (MO) basis set in then a considerable concern".
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Indeed, the problem is that, unless the system investigated is small enough so as to allow for a full CI treatment, truncated CI expansions have to be used and, according to the qualified statement ofīerthier et al. In addition, these methods, and in particular those of multiconfiguration self-consistent field (MCSCF) and CI type, are sophisticated and in virtually no case they can be used as black boxes. However, the computational challenge issued by these post-Hartree-Fock calculations is generally a formidable task, as for both variational configuration interaction (CI) and sizeconsistent many-body perturbation theory (MBPT) techniques, the amount of computations required to reach chemical accuracy is enormous. It is well known that the traditional ab initio techniques of quantum chemistry are able to incorporate many-electron effects through expansions of the many-particle wavefunction, which leads in principle to systematic procedures to take correlation effects into account. Université de Genève, Département of Chimie Physique, 30 quai Ernest-Ansermet, 1211 Genève 4, Switzerland How Much Correlation Can We Expect to Account for in Density Functional Calculations ? Case Studies of Electrostatic Properties of Small Molecules Schulman, Techniques and Applications of Path Integration, J. Binder, in International Review of Science, Physical Chemistry Series Two, Hibbs, Quantum Mechanics and Path Integrals, McGrawHill, New York (1965).Ģ8.
Gross, Density Functional Theory, Springer Verlag,ĩ.
Ludeña, Energy Density Functional Theory of ManyElectron Systems, Kluwer Academic Publ., Dordrecht (1990).Ĩ. Yang, Density-Functional Theory of Atoms and Molecules, Oxford U.P., New York (1989).ħ. C.E.Dykstra, Ab Initio Calculation of the Structures and Properties of Molecules,ĥ. Viticoli (Eds.), Materials for Photonic Devices, World Scientific, Singapore (1991).Ĥ. Prasad and D.J.Williams, Introduction to Nonlinear Optical Effects in Molecules & Polymers, J. This work was developed under the CNR "Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate" and MURST "40%" funds.ġ. University and Scientific and Technological Research (MURST) is acknowledged. Financial support by the Italian National Research Council (CNR) and by the Ministry of