Configuration interaction 2 levels

Nobes, Pople, Radotn, Handy and Knowles have studied the convergence of the Moller-Plesset orders in some detail. They computed the energies of hydrogen cyanide, cyanide anion and cyano radical through order 24 as well as at the full Configuration Interaction level. Here are some of their results ... 

The precise quantum cluster calculations of the electronic structure of SC ceramics were performed in Refs. [13,17,21]. Guo et al. [13] used the generalized valence bond method, Martin and Saxe [17] and Yamamoto et al. [21] performed calculations at the configuration interaction level. But in these studies the calculations were carried out for isolated clusters, the second aspect of the ECM scheme, see above, was not fulfilled. The influence of crystal surrounding may considerably change the results obtained. 

The present work treats the adsorption of CH, CH and H on a Ni(lll) surface in the context of a many-electron theory that permits the accurate computation of molecule-solid surface interactions at an initio configuration interaction level. The adsorbate and local surface region are treated as embedded in the remainder of the lattice electronic distribution which is modelled as a 26-atom, three layer cluster, extracted from a 62-atom cluster by an orbital localization transformation. 

One of the simplest van der Waals complexes is the helium dimer. The small size of the system has made it possible to evaluate the He- He spin-spin coupling constant in an accurate manner, at the full configuration interaction level The Fermi-contact term has been found to have non-negligible value of 1.3 Hz at R=5.6 a.u. (dose to the energy minimum), while the other contributions are practically zero. The coupling decreases very fast, in an exponential manner, with the intemuclear distance R. For R equal to 4 a.u. it is over 22 Hz, while for R over 7 a.u. it falls below... 

DHF calculations on molecules using finite basis sets require considerably more computational effort than the corresponding nonrelativistic calculations and cause several problems due to the presence of the Dirac one-particle operator. It is therefore desirable to find (approximate) relativistic Hamiltonians for many-electron systems which are not plagued by unboundedness from below and therefore do not cause problems like the variational collapse at the self-consistent field level or the Brown-Ravenhall disease at the configuration interaction level. It is also desirable to find forms in which the quality of a matrix representation of the kinetic energy is more stable than for the Dirac Hamiltonian, i.e., forms which are not affected by the finite basis set disease . 

Figure 1 Bond dissociation potential for H2 computed with RHF, UHF, PUHF (projected UHF), RMP2 (restricted MP2), UMP2 (unrestricted MP2), PMP2 (projected MP2), and full configuration interaction levels of theories using a minimal basis set. Typical RHF and UHF orbitals are shown in the insets the UHF bonding orbitals can be written as a linear combination of the RHF bonding and antibonding orbitals (equation 3) (adapted with permission from Ref. 12)...

In the RISM-SCF theory, the statistical solvent distribution around the solute is determined by the electronic structure of the solute, whereas the electronic strucmre of the solute is influenced by the surrounding solvent distribution. Therefore, the ab initio MO calculation and the RISM equation must be solved in a self-consistent manner. It is noted that SCF (self-consistent field) applies not only to the electronic structure calculation but to the whole system, e.g., a self-consistent treatment of electronic structure and solvent distribution. The MO part of the method can be readily extended to the more sophisticated levels beyond Hartree-Fock (HF), such as configuration interaction (Cl) and coupled cluster (CC). 

Each cell in the chart defines a model chemistry. The columns correspond to differcni theoretical methods and the rows to different basis sets. The level of correlation increases as you move to the right across any row, with the Hartree-Fock method jI the extreme left (including no correlation), and the Full Configuration Interaction method at the right (which fuUy accounts for electron correlation). In general, computational cost and accuracy increase as you move to the right as well. The relative costs of different model chemistries for various job types is discussed in... 

Practical configuration interaction methods augment the Hartree-Fock by adding only a limited set of substitutions, truncating the Cl expansion at some level of substitution. For example, the CIS method adds single excitations to the Hartree-Fock determinant, CID adds double excitations, CISD adds singles and doubles, CISDT adds singles, doubles, and triples, and so on. 

Craig, D. P., Proc. Roy. Soc. [London) A202, 498, Electronic levels in simple conjugated systems. I. Configuration interaction in cyclobutadiene. (ii) All the interelectron repulsion integrals, three- and four-centered atomic integrals, are included. 

Parr, R. G., Craig, D. P., and Ross, I. G., J. Chem. Phys. 18, 1561, Molecular orbital calculations of the lower excited electronic levels of benzene, configuration interaction included." One of the most complete nonempirical calculations concerning n electrons. 

Moser, C. M., and Lef bvre, R., J. Chem. Phys. 23, 598, "Lowest singlet excited levels of C10HS." Limited configurational interaction. 

The K-matrix method is essentially a configuration interaction (Cl) performed at a fixed energy lying in the continuum upon a basis of "unperturbed funetions that (at the formal level) includes both diserete and eontinuous subsets. It turns the Schrodinger equation into a system of integral equations for the K-matrix elements, which is then transformed into a linear system by a quadrature upon afinite L basis set. 

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