Ab initio wave functions

Semi-empirical Options dialog box and you can also extend the calculation of the Hartree-Fock ab initio wave function by choosing Cl in the Ab Initio Options dialog box. Use Cl for these electron configurations ... 

Pseudo-Newton-Raphson methods have traditionally been the preferred algorithms with ab initio wave function. The interpolation methods tend to have a somewhat poor convergence characteristic, requiring many function and gradient evaluations, and have consequently primarily been used in connection with semi-empirical and force field methods. 

The fact that features in the total electron density are closely related to the shapes of the HOMO and LUMO provides a much better rationale of why FMO theory works as well as it does, than does the perturbation derivation. It should be noted, however, that improvements in the wave function do not necessarily lead to a better performance of the FMO method. Indeed the use of MOs from semi-empirical methods usually works better than data from ab initio wave functions. Furthermore it should be kept in mind that only the HOMO orbital converges to a specific shape and energy as the basis set is... 

F(r) was also computed from ab initio wave functions in the framework of the HF/SCF method using 3-21G and 6-31G basis sets due to the large size ofLR-B/081, the calculation has as yet been performed on isolated molecular fragments, adopting a geometry based on molecular dimensions from X-ray diffraction studies. 

Equation (2) was also used to calculate quantum chemical approach. On the basis of previous results [19], calculated electrostatic potentials were computed from ab initio wave functions obtained in the framework of the HF/SCF method using a split-valence basis set (3-21G) and a split-valence basis set plus polarisation functions on atoms other than hydrogen (6-31G ). The GAUSSIAN 90 software package [20] was used. Since ab initio calculations of the molecular wave function for the whole... 

Once computed on a 3D grid from a given ab initio wave function, the ELF function can be partitioned into an intuitive chemical scheme [30], Indeed, core regions, denoted C(X), can be determined for any atom, as well as valence regions associated to lone pairs, denoted V(X), and to chemical bonds (V(X,Y)). These ELF regions, the so-called basins (denoted 2), match closely the domains of Gillespie s VSEPR (Valence Shell Electron Pair Repulsion) model. Details about the ELF function and its applications can be found in a recent review paper [31],... 

As all of the terms in the effective ESR Hamiltonian correspond to quantities observable experimentally through an energy splitting between quantum mechanical states, different quantum chemical protocols exist to calculate such splittings with ab initio wave-function methods or DFT (63,65-79). 

Meerts and Dymanus [142] compared their measured hyperfine constants with those calculated from ab initio wave functions for both OH and SH. There are a series of direct relations between the four hyperfine constants and the following average quantities calculated from an electronic wave function ... 

The magnetic hyperfine parameters have been defined in a number of different places in this book. Levy and Hinze [191] have used the single configuration Hartree-Fock ab initio wave function calculated by Lie, Hinze and Liu [ 188] to calculate the Frosch and Foley [192] magnetic hyperfine constants. The results, compared with experiment, are as follows ... 

Using ab initio wave functions and the full operators, dipole and second moments (cf. Section 4.11.3.2.2) as well as electronic charge distributions have been calculated for... 

Pakiari and Noorizadeh have suggested an ab initio determination of a rather accurate force constant for given FSGO wavefunctions for LiH, BH, H2 and H2O. In this procedure, an th degree polynomial must be fitted to dilferent calculated potential points from ab initio wave function. Brief results are collected in Table 6. 

We end this short review on ab initio wave function-based methods by noting all of them can be applied to a cluster model, whereas only the Hartree-Fock can be extended to account for periodic symmetry [68]. 

At the same time, the derivative of the coupling can also be obtained from the ab initio wave functions. 

The quantum chemist s traditional way to approximate solutions of the electronic Schrodinger equation is so-called ab initio, wave function-based electron correlation methods. These methods improve upon the HF mean-field approximation by adding many-body corrections in a systematic way [15]. As of the time of this writing, efforts to accelerate ab initio calculations with GPUs are scarce. However, it is expected that this will change in the near future because these methods are of critical importance whenever higher accuracy is required than what can be achieved by DFT or for types of interactions and properties for which DFT breaks down. 

Qm QM RECON Mean absolute atomic charge Quantum mechanics An algorithm for the rapid reconstruction of molecular charge densities and charge density-based electronic properties of molecules, using atomic charge density fragments precomputed from ab initio wave functions. The method is based on Bader s quantum theory of atoms in molecules. 

Mayer, 1. (1986a) Bond orders and valences from ab initio wave functions. Int. J. Quant. Chem., 29, 477-483. 

Most of the general methods developed for isolated molecules have been used to treat PH as supermolecules HF, Cl, CEPA, MPn, MRD-CI, CC, DFT, etc... They can use semi-empirical or, more often, ab initio wave functions. Let us notice that MP2 is mostly used in the last decade. Among the emerging methods, let us mention CASPT2 calculations on the excited states of H30+(59i) and DFT studies more or less arbitrarily included in quantum chemistry treatments. VB calculations are found in only one case. 

TABLE 4 Predicted Bond Distances (A) for HF and F2 from a Range of ab initio Wave Functions... 

As already noted, all the examples in the preceding section refer to SCF ab initio wave functions, which do not take into account electron correlation. It is well known, however, that the SCF approximation at the Hartree-Fock limit is good enough to give a reliable representation of a one-electron, first-order observable like the electronic potential, at least for closed-shell ground-state systems like those considered here. If we keep to the field of SCF ab initio wave functions, the differences in accuracy between several wave functions for the same molecule depend upon the adequacy of the expansion basis set / employed in the calculations. In the next paragraph, however, we will also treat the case of semiempirical SCF wave functions. 

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