Valence complete active space self-consistent field

Multireference configuration interaction (MRCI) calculations based on valence complete active space self-consistent field (CASSCF) wave functions with the quadruples corrections (-I-Q) [25-30] were used to determine the excitation energies of the N and P atoms using the AVQZ basis set. 

Table 11 Comparison of De, re, and o>c from Hartree-Fock (HF) and Valence Complete Active Space Self-consistent Field (val-CASSCFO Calculations on Carbon Monoxide With the cc-pVnZ Basis Sets. Experimental Data from Ref. 72...

The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ringopening of cyclohexadiene. 

Amovilli et al. [20] presented a method to carry out VB analysis of complete active space-self consistent field wave functions in aqueous solution by using the DPCM approach [3], A Generalized Valence Bond perfect pairing (GVB-PP) level... 

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. 

The complete active space valence bond (CASVB) method is an approach for interpreting complete active space self-consistent field (CASSCF) wave functions by means of valence bond resonance structures built on atom-like localized orbitals. The transformation from CASSCF to CASVB wave functions does not change the variational space, and thus it is done without loss of information on the total energy and wave function. In the present article, some applications of the CASVB method to chemical reactions are reviewed following a brief introduction to this method unimolecular dissociation reaction of formaldehyde, H2CO — H2+CO, and hydrogen exchange reactions, H2+X — H+HX (X=F, Cl, Br, and I). 

Bernardi, F., Olivucci, M., McDouall, J.J.W. and Robb, M.A. (1988) Parameterization of a Heitler-London Valence Bond Hamiltonian from Complete-Active-Space Self-Consistent-Field Computations An Application to Chemical Reactivity, J. Chem. Phys. 89, 6365-6375. 

Valence bond description of complete active space self-consistent field function... 

The wave function of fragment A, ll a, can either be a single determinant from HF theory or Kohn-Sham DFT, or a multiconfiguration wave function derived from complete active space self-consistent field (CASSCF) or valence bond (VB) calculations. 

Blancafort L, Robb MA. A valence bond description of the prefulvene extended conical intersection seam of benzene. J Chem Theory Comput. 2012 8 4922-4930. Blancafort L, Celani P, Beatpark M, Robb M. A valence-bond-based complete-active-space self-consistent-field method for the evaluation of bonding in organic molecules. Theor Chem Acc. 2003 110 92-99. 

As an illustration of the performance of TDDFT, we compare various density functionals and wave function methods for the first singlet excited states of naphthalene in Tables 4, 5, and 6. All calculations were performed using the aug-TZVP basis set, while the complete active space self-consistent field (SCF) with second-order perturbation theory (CASPT2) results from Ref. 200 were obtained in a smaller double-zeta valence basis set with some diffuse augmentation. The experimental results correspond to band maxima from gas-phase experiments however, the position of the band maximum does not necessarily coincide with the vertical excitation energy, especially if... 

---