Complete active space self-consistent fields electronic structure

Roos B O 1987 The complete active space self-consistent field method and its applications in electronic structure calculations Adv. Chem. Phys. 69 399-445... 

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... 

K. P. Lawley, Ed., chapter 69,399. John Wiley Sons Ltd., Chichester, England, 1987. The Complete Active Space Self-Consistent Field Method and its Applications in Electronic Structure Calculations. 

B. O. Roos, The Complete Active Space Self-Consistent Field Method and Its Application in Electronic Structure Calculation, Volume 69 olAdvances in Chemical Physics, Wiley, Chichester, 1987, p. 399. 

B. O. Roos. The complete active space self-consistent field method and its apphca-tion in electronic structure calculations. In K. P. Lawley, editor, Ab Initio Methods in Quantum Chemistry. Part II, volume 69 of Adv. Chem. Phys., pages 399-446. John Wiley, Chichester, 1987. 

B.O.Roos, The Complete Active Space Self-Consistent Field Method and its Application in Electronic Structure Calculations. 

To obtain geometries, 10-orbital 10-electron complete active space self-consistent field (CASSCF) [82-84] calculations were performed with the GAMESS-UK program [6], The occupied orbital order in an SCF for flat benzene is n,2c,2n. In the bent molecule, there is no clear distinction between a- and tt-orbitals and we want to include all the tt-orbitals in the CAS-space. Thus, 10 orbitals in the active space are required. Obviously, the 5 structure VB wavefunction would have been a preferable choice to use in the geometry optimisation. However, at that time, the VB gradients were not yet available. The energies of the VBSCF at the CASSCF geometries followed the CASSCF curve closely. 

For the construction of spin eigenfunctions see, for example, Ref. [22], There are obviously many parallels to the multiconfiguration self-consistent field (MCSCF) methods of MO theory, such as the restriction to a relatively small active space describing the chemically most interesting features of the electronic structure. The core wavefunction for the inactive electrons, 4>core, may be taken from prior SCF or complete active space self-consistent field (CASSCF) calculations, or may be optimised simultaneously with the and cat. 

THE COMPLETE ACTIVE SPACE SELF-CONSISTENT FIELD METHOD AND ITS APPLICATIONS IN ELECTRONIC STRUCTURE CALCULATIONS... 

In this section, we briefly review the use of the complete active space self-consistent field (CASSCF) method for calculating excited states. This method offers an acceptable compromise between accuracy and computational expense, but our main reason for choosing it is that it offers analytical gradients and second derivatives, which are essential for geometry optimization. As we discuss more fully below, CASSCF is often sufficient if one is interested in structure and mechanism (as we are here), but a more accurate treatment of dynamic electron correlation is often necessary for accurate energetics. 

More recently, the influence on the Bi(ai bj) (n,7t ) excitation of 1 in water was studied starting from ab initio CASSCF (complete active space self consistent field) estimates of the gas-phase electronic excitation properties, followed by Monte Carlo simulations to elucidate the structures of the liquid around the ground and excited state solute. Finally, the solvent shift was evaluated based on gas-phase charge distributions and solvent structures. One linear H-bond to each N-atom of 1 is predicted for diluted solutions <2003CPL(368)377, 2004JCC813>, and three H-bonds to the ground state... 

The electronic wavefunction was evaluated using the state averaged complete active space self-consistent field (SA-CASSCF) method [49, 50], as implemented in the Molpro [51] electronic structure package. We used the 6-31G basis set [52] and an active space of two electrons in two orbitals with equally weighted averaging over the lowest three singlet states, i.e., SA-3-CAS(2/2)/6-31G. ... 

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