Full Configuration-Interaction Calculations

A CASSCF calculation is a combination of an SCF computation with a full Configuration Interaction calculation involving a subset of the orbitals. The orbitals involved in the Cl are known as the active space. In this way, the CASSCF method optimizes the orbitals appropriately for the excited state. In contrast, the Cl-Singles method uses SCF orbitals for the excited state. Since Hartree-Fock orbitals are biased toward the ground state, a CASSCF description of the excited state electronic configuration is often an improvement. 

The problem is also a challenge from both group theoretical [6,7] and experimental [8] point of view. In the following we will use a method which is based on a multipole expansion of the Coulomb interaction between electrons on a same molecule [9,10]. Thereby we systematically include electronic transitions which go beyond the usual Hartree-Fock scheme and hence our approach is equivalent to a full configuration interaction calculation. The details of our technique are given in Ref. [10]. 

R.L. Graham, D.L. Yeager, J. Olsen, P. Jorgensen, R. Harrison, S. Zarrabian, R. Bartlett, Excitation energies in Be A comparison of multiconfigurational linear response and full configuration interaction calculations, J. Chem. Phys. 85 (1986) 6544. 

It has been shown that the finite difference HF orbitals can be augmented with the virtual ones to form numerical basis sets for the full configuration interaction calculations. In the case of the beryllium and the... 

Knowles, J. Chem. Phys., 85, 1469 (1986). Benchmark Full Configuration Interaction Calculations on HF and NH2. 

When all the Slater determinants that can be constructed from a set of Hartree-FockMOs are taken into account, the method is called full configuration interaction. It provides the best possible solutions of the non-relativistic, clamped nuclei TISE within the basis set used for the calculation. The exact solutions are thus obtained from a full configuration interaction calculation at the complete basis limit. However, the number of configurations that can be built from a set of MOs grows extremely fast with the size of the system (i.e the size of the molecule and/or of the basis set). 

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