CASSCF MCSCF theory

CASSCF is a version of MCSCF theory in which all possible configurations involving the active orbitals are included. This leads to a number of simplifications, and good convergence properties in the optimization steps. It does, however, lead to an explosion in the number of configurations being included, and calculations are usually limited to 14 elections in 14 active orbitals. 

MCSCF theory is a specialist branch of quantum modelling. Over the years Jt has become apparent that there are computational advantages in treating all oossible excitations arising by promoting electron(s) from a (sub)set of the occu-orbitals to a (sub)set of the virtual orbitals. We then speak of complete active ace MCSCF, or CASSCF. 

Multiconfiguration SCF (MCSCF) Theory and Complete Active Space SCF (CASSCF) Theory... 

The deviation of the CASSCF curve from the FCI curve in Fig. 2 is caused by nonstatic or dynamical correlation [1]. Although dynamical correlation is usually less geometry-dependent than static correlation, it must be included for high accuracy (see Sec. 4). One might think that it is possible to include the effects of dynamical correlation simply by extending the active space. For small molecules, this is, to some extent true, in particular when using the techniques of restricted active space SCF (RASSCF) theory [46]. Nevertheless, because of the enormous number of determinants needed to recover dynamical correlation, the simultaneous optimization of orbitals and configuration coefficients as done in MCSCF theory is not a practical approach to the accurate description of electronic systems. 

CASSCF is a variant of multi-configuration self-consistent field (MCSCF) theory. This means that in addition to the Cl expansion coefficients being varia-tionally optimized, the orbitals determining the expansion are also variationally optimized. Thus, the linear combination of atomic orbitals (LCAO) coefficients defining the orbitals is simultaneously optimized. If one starts with, for example, Hartree-Fock orbitals, then after the MCSCF wavefunction is optimized the orbitals will (often) be quite different. MCSCF wavefunctions thus contain the optimum orbitals for the given Cl expansion. CASSCF involves choosing a subset... 

To use direct dynamics for the study of non-adiabatic systems it is necessary to be able to efficiently and accurately calculate electronic wave functions for excited states. In recent years, density functional theory (DFT) has been gaining ground over traditional Hartree-Fock based SCF calculations for the treatment of the ground state of large molecules. Recent advances mean that so-called time-dependent DFT methods are now also being applied to excited states. Even so, at present, the best general methods for the treatment of the photochemistry of polyatomic organic molecules are MCSCF methods, of which the CASSCF method is particularly powerful. 

In some points of the previous analysis we have used formulations based on the Hartree-Fock (HF) expression of the quantum problem, mainly for simplicity of exposition. As a matter of fact, there are no formal reasons to limit continuum solvent approach to the HF level. Actually, PCM solvation procedures have been extended to MCSCF (Aguilar et al., 1993b), Cl (Persico and Tomasi, 1984), MBPT (Olivares del Valle et al., 1991, 1993), CASSCF, MR-SDCI (Aguilar et al., 1993b), DFT (Fortunelli and Tomasi, 1994) levels of the quantum description. The other continuum solvation methods have, at least in principle, the same flexibility in the definition of the quantum theory level to be used in computations. 

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. 

Multiconfigurational quasi-degenerate perturbation theory (MC-QDPT) [5,6] We have also proposed a multistate multireference perturbation theory, the QDPT with MCSCF reference functions (MC-QDPT). In this PT, state-averaged CASSCF is first performed to set reference functions, and then an elfective Hamiltonian is constructed, which is finally diagonalized to obtain the energies of interest. 

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