Computational methods CASSCF

It is important to recognize that the low level computation, and the low and intermediate level computations in a three-layer ONIOM, does not have to be MM. Any computational method can be used for any layer. For example, a three-layer ONIOM might use a semiempirical computation for the real system (5q), a DPT computation for the intermediate layer (Si), and a CASSCF computation for the high layer (82). 

The list of unsuccessful computational methods for the calculation of vibrational wave numbers for N2 and SiN2 (la) was extended from HF, CASSCF, and CISD to MP2. Density functional theory (DFT) in the B3LYP framework yielded vibrational wave numbers 5% above experimental values throughout. Only CCSD(T) results are closer to the experiment (6), but the B3LYP method is a better candidate for the investigation of larger systems. 

How can we extend HF theory to incorporate the effects of the most important natural orbitals, even in cases where the occupation numbers are not close to two or zero Actually, Lowdin gave an answer to this question in his 1955 article, where he derived something he called the extended HF equations. The idea was to use the full Cl wave fimction. Equation (2), but with a reduced number of orbitals, and determine the expansion coefficients and the molecular orbitals variationally. His derivation was formal only and had no impact on the general development at the time. It was not xmtil 20 years later that a similar idea was suggested and developed into a practical computational procedure. The approach is today known as the complete active space SCF method, CASSCF. ... 

Gimer V, KhuongKS, Leach AG, Lee PS, BartbergerMD, HoukKN A standard set of peri-cyclic reactions of hydrocarbons for the benchmarking of computational methods the performance of ab initio, density functional, CASSCF, CASPT2, and CBS-QB3 methods for the prediction of activation barriers, reaction energetics, and transition state geometries, / Phys Chem A 107(51) 11445—11459, 2003. 

If the CASSCF wavefunction is reasonable, CAS-CISD always gives very accurate predictions. Thus, we may regard the CAS-CISD results in Table 1 as benchmarks against which the results of other computational methods may be judged. Because the cost of CAS-CISD calculations grows very rapidly with the size of the active space and the molecule, however, the use of this method is frequently impractical. For example, the CAS-CISD calculations on benzyl in Table 1 had to be performed with an active space of five electrons in five orbitals, rather than with the full active space, containing all the Ji electrons, which was used in the (7/7)CASSCF calculations. 

Field (CASSCF) Second-order Perturbation Theory (CAS-PT2) Configuration Interaction Core-Valence Correlation Effects Coupled-cluster Theory Experimental Data Evaluation and Quality Control G2 Theory Heats of Formation Isoelectronic Isogyric Reactions M0ller-Plesset Perturbation Theory Numerical Hartree-Fock Methods for Molecules r 12-Dependent Wavefunctions Relativistic Theory and Applications Spectroscopy Computational Methods Spin Contamination Transition Metals Applications,... 

Combined Quantum Mechanics and Molecular Mechanics Approaches to Chemical and Biochemical Reactivity Complete Active Space Self-consistent Field (CASSCF) Second-order Perturbation Theory (CASPT2) Density Functional Applications Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field Density Functional Theory Applications to Transition Metal Problems Quantum Mechanical/Molecular Mechanical (QM/MM) Coupled Potentials Spectroscopy Computational Methods Transition Metals Applications. 

The boat-type transition state has also been studied using computational methods. All methods predict substantially looser transition structures. Again the MPn and local DFT methods yield shorter Cl -C6 bond lengths, ranging from 1.97 to 2.14 A. The RHF and CASSCF as well as the nonlocal and hybrid DFT calculations yield looser transition structures... 

The energies of this Cl and of the other ones calculated in this work are listed in Table III. The calculated CASSCF values of the energies of the two lowest electronically states are 9.0 eV (5i, vertical) and 10.3 eV ( 2, vertical) [99]. They are considerably higher than the expenmental ones, as noted for this method by other workers [65]. In all cases, the computed conical intersections lie at much lower energies than the excited state, and are easily accessible upon excitation to Si. In the case of the H/allyl Cl, the validity confirmation process recovered the CHDN and 1,3-CHDN anchors. An attempt to approach the third anchor [BCE(I)] resulted instead in a biradical, shown in Figure 43. The bhadical may be regarded as a resonance hybrid of two allyl-type biradicals. 

In this exercise, we will introduce the Complete Active Space Multiconfiguration SCF (CASSCF) method, using it to compute the excitation energy for the first excited state of acrolein (a singlet). The CIS job we ran in Exercise 9.3 predicted an excitation energy of 4.437 eV, which is rather for from the experimental value of 3.72 eV. We ll try to improve this prediction here. 

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. 

In a combined experimental/computational study, the vibrational spectra of the N9H and N7H tautomers of the parent purine have been investigated [99SA(A) 2329]. Solvent effects were estimated by SCRF calculations. Vertical transitions, transition dipole moments, and permanent dipole moments of several low-lying valence states of 2-aminopurine 146 were computed using the CIS and CASSCF methods [98JPC(A)526, 00JPC(A)1930]. While the first excited state of adenine is characterized by an n n transition, it is the transition for 146. The... 

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