Valence, full, CASSCF calculation

We will now look at how different types of wave functions behave when the O-H bond is stretched. The basis set used in all cases is the aug-cc-pVTZ, and the reference curve is taken as the [8, 8J-CASSCF result, which is slightly larger than a full-valence Cl. As mentioned in Section 4.6, this allows a correct dissociation, and since all the valence electrons are correlated, it will generate a curve close to the full Cl limit. The bond dissociation energy calculated at this level is 122.1 kcaPmol, which is comparable to the experimental value of 125.9 kcal/mol. 

The multireference results of Table 5.12 were all based on full valence CASSCF calculations with eleven electrons in twelve active orbitals. This produces a large configuration expansion (about 85 000 CSFs) so it is not possible to perform MRCI(CAS) calculations. Reference configuration lists were selected at the cyclic and linear geometries (taken from MP2 optimized structures) and then merged. The core electrons were not correlated in any of the calculations. One complication in the CASSCF calculations should be pointed out. Since the cyclic state arises as the 2B component, in C3v symmetry, of a 2E state in the D3h symmetry (equilateral triangular) structure, it would be desirable to obtain MOs with D3h symmetry and equivalence restrictions... 

For a system with N electrons, a spin-coupled calculation followed by a full-valence VB calculation produces a total wavefunction which is formally equivalent to a CASSCF description with all allowed distribution of N electrons in N orthogonal orbitals. Robb and co-workers [15] have shown how in principle one might transform between these two descriptions. The interpretation of the CASSCF wavefunction for benzene is not straightforward, as many of the 175 configurations make significant... 

Nondynamical electron correlation effects are generally important for reaction path calculations, when chemical bonds are broken and new bonds are formed. The multiconfiguration self-consistent field (MCSCF) method provides the appropriate description of these effects [25], In the last decade, the complete active space self-consistent field (CASSCF) method [26] has become the most widely employed MCSCF method. In the CASSCF method, a full configuration interaction (Cl) calculation is performed within a limited orbital space, the so-called active space. Thus all near degeneracy (nondynamical electron correlation) effects and orbital relaxation effects within the active space are treated at the variational level. A full-valence active space CASSCF calculation is expected to yield a qualitatively reliable description of excited-state PE surfaces. For larger systems, however, a full-valence active space CASSCF calculation quickly becomes intractable. 

It is known that, in the MO framework, the nondynamical electron correlation is accounted for by means of a so-called CASSCF calculation, which is nothing else than a full Cl in a given space of orbitals and electrons, in which the orbitals and the coefficients of the configurations are optimized simultaneously. If the active space includes all the valence orbitals and electrons, then the totality of the nondynamical correlation of the valence electrons is accounted for. In the VB framework, an equivalent VB calculation, defined with pure AOs or purely localized hybrid atomic orbitals (HAOs), would involve all the covalent and ionic structures that may possibly be generated for the molecule at hand. Note that the resulting covalent—ionic VB wave function would have the same dimension as the valence—CASSCF one (e.g., 1764 VB structures for methane, and 1764 MO SCF configurations in the CASSCF framework). 

Two sets of preliminary CASSCF calculations were preformed in order to test the above assumption. A double-zeta (DZ) AO basis was used. One calculation was made with a full valence active space, while the two orbitals 4ffg and 4second calculation. Some of the results of these calculations are reported in Table II. The addition of the two extra valence orbitals had a very small effect on the potential curve for the state, but the Tj value of the state dropped from 0.55 to 0.29 eV with the addition of these active orbitals. The rather poor overall agreement with experiment obtained in these calculations is, of course, due to the limited basis set used, but they illustrate the importance of the 4a orbital for a balanced description of the two potential curves. 

Scheme 2 Orbital diagram of triplet Pdj and Pt2,MO energies (in hartree) are calculated at the full valence (20el./12orb.) CASSCF level...

The CV correlation contributions were computed at the MRCI + Dav level of theory using the cc-pwCVQZ basis sets, where the CASSCF reference states were obtained by employing an active space of six electrons in six active orbitals. Unfortunately, MRCI calculations with a full-valence CASSCF reference state with all electrons correlated proved to be too demanding computationally, hence... 

The geometry of HeBN has also been optimized using full-valence CASSCF wave functions with a 6-31G (d,p) basis set. The optimization was stopped at a B-He distance of 3.05 A and at a B-N distance of 1.292 A. This leads to the assumption that He is bound to B-N by dipole-induced dipole forces. The equilibrium distances for ArBN have been calculated at the MP3/6-31G (d,p) level to be r(ArB)=1.847 A and r(BN) =1.250 A [62]. 

Bauschlicher and Walch (1982) have considered full valence MCSCF/CI calculations on the lowest six electronic states of ScH. Subsequently, Chong et al. (1986) have studied the dipole moments of the first-row hydrides using the MCPF and CASSCF/MRCI techniques. 

Das and Balasubramanian (1991c) have computed the spectroscopic constants and potential-energy curves of LaH. They used a C ASSCF method followed by full second-order Cl calculations in conjugation with 5s 5p 5d6s RECPs of Ross et al. (1990). The basis set used was (5s5p3d) valence Gaussian basis set. At both CASSCF and SOCI levels, excitations from the 5s 5p shells were not allowed. The CASSCF calculations... 

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