Restricted active space RASSCF

Restricted active space self-consistent field (RASSCF) scheme, ab initio calculations, P,T-odd interactions, 253-259... 

The selection of configuration state functions to be included in MCSCF calculations is not a trivial task. Two approaches which can reduce the complexity of the problem are the complete active space self-consistent-field (CASSCF) [68] and the restricted active space self-consistent-field (RASSCF) [69] approach. Both are implemented in the Dalton program package [57] and are used in this study. Throughout the paper a CASSCF calculation is denoted by i active gactive RASSCF calculation by For the active spaces of HF, H2O, and CH4... 

As for any full Cl expansion, the CASSCF becomes unmanageably large even for quite small active spaces. A variation of the CASSCF procedure is the Restricted Active Space Self-consistent Field (RASSCF) method. Here the active MOs are further divided into three sections, RASl, RAS2 and RAS3, each having restrictions on the... 

RASSCF see restricted-active-space selfconsistent field... 

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. 

Correlation Restricted Active Space Self-Consistent Field (RASSCF) and Multiconfigurational Second-Order Perturbation Theory (CASPT2)... 

Empirical equilibrium coupling constants can be compared as a benchmark with calculated equilibrium coupling constants obtained with various methods. A comparison of these empirical equilibrium constants with calculated equilibrium constants suggested that the restricted-active-space self-consistent field (RASSCF) method is the best approach for calculating the indirect nuclear spin-spin coupling constants of small molecules, and that the second-order polarization propagator approximation (SOPPA) and DFT are similar in performance. 

More precisely, a set of active orbitals and a number of active electrons are defined. All the configurations that can be built by excitations ofthe active electrons among the active orbitals are included into the wavefunction. Other, more flexible approaches for the selection of the conflgurations included in the MSCSF wavefunction exist, such as the Restricted Active Space Self-Consistent Field [23, 24] (RASSCF) and the Occupation Restricted Multiple Active Space (ORMAS) [25] methods. 

Instead of optimization of all the orbitals, only the active orbitals will be varied within the Complete Active Space Self-Consistent Field approximation (CASSCF). In the acronym of this method, the number of active orbitals and active electrons are also provided for the given system. For instance, CASSCF(6,4) denotes the calculations with the expansion including all the possible exchanges of the four electrons within the six active orbitals. The CASSCF approach leads to all possible exchanges in the given active space, and for a moderately sized system, the size of the active spaces can quickly exceed the computational resources. In such a case, the solution can be the Restricted Active Space Self-Consistent Field method (RASSCF), which supplies a way of limiting the size of the active space. 

On the other hand, the Restricted Active Space Self-Consistent Field (RASSCF) method is a more general extension of the CASSCF method. Now, there are three subspaces within the active orbitals RASl (orbitals that are doubly occupied except for a maximum number of holes allowed in this orbital subspace), RAS2 (in these orbitals, all possible occupations are allowed), and RAS3 (orbitals that are unoccupied except for a maximum number of electrons allowed in this subspace). CASSCF calculations can be performed by allowing orbitals only in the RAS2 space. A single reference SDCl wave function is obtained by allowing a maximum of two holes... 

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