Complete active space self-consistent field geometries

Clifford and co-workers [190] have performed complete active space self-consistent field (CASSCF) ab initio calculations on the photocycloaddition reactions of benzene and ethene. An eight-electron, eight-orbital active space involving the ir-orbitals of the benzene and ethene moieties was used. The geometries were optimized using the 4-31G basis set, and the energies were recomputed at the 6-31G level. 

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. 

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. 

Our group has already studied all the fluoro-, chloro-and bromo-carbenes [11-16]. We used the complete active space self-consistent field (CASSCF) method as well as complete active space second-order perturbation theory (CASPT2) and multi-reference configuration interaction (MRCI) approaches to compute the geometries, force constants, and vibrational frequencies of the (singlet) X and A states as well as the (triplet) a states. Our theoretical studies of most of these carbenes were carried out specifically to complement LIF studies that were pursued in our laboratories by Kable et al. [6]. In addition to the determination of spectroscopic constants, the spectroscopic and theoretical studies considered dynamics on the A surfaces, i.e. whether photodissociation or internal conversion to the ground state would occur. 

Basis Sets Correlation Consistent Sets Complete Active Space Self-consistent Field (CASSCF) Second-order Perturbation Theory (CASPT2) Configuration Interaction Coupled-cluster Theory Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field G2 Theory Geometry Optimization 1 Gradient Theory Inter-molecular Interactions by Perturbation Theory Molecular Magnetic Properties NMR Chemical Shift Computation Ab Initio NMR Chemical Shift Computation Structural Applications Self-consistent Reaction Field Methods Spin Contamination. 

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