CASSCF calculations

For this reason, there has been much work on empirical potentials suitable for use on a wide range of systems. These take a sensible functional form with parameters fitted to reproduce available data. Many different potentials, known as molecular mechanics (MM) potentials, have been developed for ground-state organic and biochemical systems [58-60], They have the advantages of simplicity, and are transferable between systems, but do suffer firom inaccuracies and rigidity—no reactions are possible. Schemes have been developed to correct for these deficiencies. The empirical valence bond (EVB) method of Warshel [61,62], and the molecular mechanics-valence bond (MMVB) of Bemardi et al. [63,64] try to extend MM to include excited-state effects and reactions. The MMVB Hamiltonian is parameterized against CASSCF calculations, and is thus particularly suited to photochemistry. 

A simple example would be in a study of a diatomic molecule that in a Hartree-Fock calculation has a bonded cr orbital as the highest occupied MO (HOMO) and a a lowest unoccupied MO (LUMO). A CASSCF calculation would then use the two a electrons and set up four CSFs with single and double excitations from the HOMO into the a orbital. This allows the bond dissociation to be described correctly, with different amounts of the neutral atoms, ion pair, and bonded pair controlled by the Cl coefficients, with the optimal shapes of the orbitals also being found. For more complicated systems... 

An MCSCF calculation in which all combinations of the active space orbitals are included is called a complete active space self-consistent held (CASSCF) calculation. This type of calculation is popular because it gives the maximum correlation in the valence region. The smallest MCSCF calculations are two-conhguration SCF (TCSCF) calculations. The generalized valence bond (GVB) method is a small MCSCF including a pair of orbitals for each molecular bond. 

A CASSCF calculation is a combination of an SCF computation with a full Configuration Interaction calculation involving a subset of the orbitals. The orbitals involved in the Cl are known as the active space. In this way, the CASSCF method optimizes the orbitals appropriately for the excited state. In contrast, the Cl-Singles method uses SCF orbitals for the excited state. Since Hartree-Fock orbitals are biased toward the ground state, a CASSCF description of the excited state electronic configuration is often an improvement. 

A CASSCF calculation is requested in Gaussian with the CASSCF keyword, which requires two integer arguments the number of electrons and the number of orbitals in the active space. The active space is defined assuming that the electrons come from as many of the highest occupied molecular orbitals as are needed to obtain the specified number of electrons any remaining required orbitals are taken from the lowest virtual orbitals. 

Perform a series of CASSCF calculations on acrolein to predict the excitation energy of its first excited state. In order to complete a CASSCF study of this excited state, you will need to complete the following steps ... 

Studies on Cg show that cyclic cumulenes are not well described by a one-determinant wavefunction. In a valence-CASSCF calculation on cyclic Cg, for example, the Hartree-Fock determinant has a weight of only 0.40. The problem is assumed to be aggravated for larger systems, as the HOMO-LUMO gap diminishes. 

Since CASSCF calculations are necessary to obtain accurate excitation energies the examination of a large number of snapshots would be intractable. The MD/QM methodology instead allows the use of a few, carefully chosen representative structures for which to carry quantum simulations. 

The possibility of a biradical mechanism was suggested using the MNDO and AMI semiempirical methods, for the addition of protoanemonin (5-methylene-2(5Z/)-furanone) to butadiene105 and to several substituted dienes106. Experimental evidence for this kind of mechanism has recently been published133. A biradical mechanism has also been considered for the dimerization of butadiene96. For this reaction, CASSCF calculations... 

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... 

The yttrium monocarbide molecule was only recently observed under high resolution by Simard et al. (37) using Jet-cooled optical spectroscopy. The ground electronic state was determined to be an 0=5/2 state, which was consistent with the ab initio calculations of Shim et al. (38) who predicted a 11 ground state for YC in CASSCF calculations. The experimental work of Simard et al. yielded estimates for both the bond length and harmonic frequency of YC. In addition to their CASSCF calculations. Shim et al. (38) also reported results from mass spectrometric equilibrium experiments, which resulted in a bond dissociation energy of Do = 99.0 3.3 kcal/mol. The results from the present work are shown in Table I. An open-shell coupled cluster singles and doubles... 

As already mentioned, choosing the active space for CASSCF calculations is not always a trivial matter. In the systems under consideration, there is a plane of symmetry (that of the phenylene linker), which helps in classifying the MOs as CT and tt (or A and A", using group-theory notation). Experience shows that a reasonably balanced active space is made of the -ir system of the linker and one CT-orbital and one -ir-orbital per reactive site (carbene or nitrene) (Fig. 2). 

Subsequent (14, 14, S) CASSCF calculations, in which we removed the previous restriction on the lbi and lb2g orbitals, were carried out for the two lowest-energy term symbols for each value of S (see Table 1). The relative CASSCF energies (A cas) are reported in Table 2 for the lowest root of each spin multiplicity. These values, which are fairly similar to those from the ten-electron SC calculations of Raos et al. [71], add further weight to their assertion that the ground state of this system should be a triplet. 

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