Self-consistent field molecular orbitals SCF

Various theoretical methods (self-consistent field molecular orbital (SCF-MO) modified neglect of diatomic overlap (MNDO), complete neglect of differential overlap (CNDO/2), intermediate neglect of differential overlap/screened approximation (INDO/S), and STO-3G ab initio) have been used to calculate the electron distribution, structural parameters, dipole moments, ionization potentials, and data relating to ultraviolet (UV), nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), photoelectron (PE), and microwave spectra of 1,3,4-oxadiazole and its derivatives <1984CHEC(6)427, 1996CHEC-II(4)268>. 

Wakatsuki et al. (83JA1907) studied the transformation of cpCo(bisal-kyne) into cp-cobaltacyclopentadiene by ab initio self-consistent field-molecular orbital (SCF-MO) calculations. For the cpCo(bisalkyne), they... 

Finally, if we abandon Hiickel s topological approach altogether and consider more elaborate quantum-mechanical approaches, the concept of aromaticity derived purely from a consideration of -electrons becomes blurred and tends to disappear completely. In fact, allelectron methods allow the calculation of aromatic properties (Section V,B) of a given substance without introducing explicitly the concept of aromaticity. Certain authors, notably Dewar,19 have published resonance energies derived from self-consistent field molecular-orbital (SCF-MO) calculations, and these could be used as a measure of aromaticity. 

Early self-consistent field molecular orbital (SCF-MO) studies on PH3, PF3, and PMe3 by Hillier and co-workers 123, 144) predicted ionization potential data which were in good agreement with the experimental values determined by UV photoelectron spectroscopy 241). More recently, the electronic structures of these phosphines have been reexamined 67, 366). Self-consistent multipolar Xa calculations (SCM-Xa-DV) by Xiao et al. 366) give excellent agreement between the theoretical and experimental ionization energies. When the transition-state procedure is used, the first ionization potentials of 10.39,8.41, and 12.19 eV for PH3, PMe3, and PF3 are calculated compared with the experimental values of 10.58, 8.58, and 12.27 eV (Fig. 10). 

Proton-transfer reactions of iV-tetrachlorosalicylideneaniline (140) and N-tetrachlorosalicylideneaniline-l-pyrenylamine (171) have been investigated by using a semiempirical self-consistent field molecular orbital (SCF MO) method with an energy gradient technique.94 From the calculated potential barriers (81.27 and 87.30kJmol-1) for 140 and 171, respectively, it can be seen that the... 

The first full sets of self-consistent-field molecular orbitals (SCF-MO) were reported by dementi [20] and dementi and McLean [21] as part of a comprehensive study of linear and symmetrical ions. A value of 1.20 A (2.27 atomic units) was used for the N-N separation. The former calculation used a set of Is, 2s, and 2p Slater functions, with the exponents derived from Roothaan s [22] best free-atom values. The latter used an additional 2po Slater-type orbital (STO) and a 3c a STO on the central nitrogen. The extension of the basis improved the total energy by approximately 4.4 eV. Comparable results were obtained by Bonaccorsi and coworkers [23] using essentially the same methods and basis functions (not included in Table I). 

Ab initio calculations are based on iterative procedures and provide the basis for self-consistent field-molecular orbital SCF-MO) methods. Electron-electron repulsion is specifically taken into account. Normally, calculations are approached by the Hartree-Fock closed-shell approximation, which treats a single electron at a time interacting with an aggregate of all the other electrons. Self-consistency is achieved in the Roothaan method by a procedure in which a set of orbitals is assumed, and the electron-electron repulsion is calculated this energy is then used to calculate a new set of orbitals, which in turn are used to calculate a new repulsive energy. The process is continued until convergence occurs and self-consistency is achieved. 

The time scale for electron dynamics studied above is very short, typically shorter than 1 femtosecond, during which the nuclei are supposed to stay still. In the above process, the electron loss has been taken into account through the change of the population of natural orbitals and subsequently the change of the coefficients of the Slater determinants. Therefore the same set of self-consistent field molecular orbitals (SCF MO) may be used for the short time period. However, for a longer time scale, it would be better to redetermine the molecular orbitals under the gradual loss of electronic population. This demands an efficient way to determine the molecular orbitals with variable and fractional occupation numbers, although this is not a new problem for quantum chemistry. 

S2 emission—See Upper excited-state emission spectroscopy Self-consistent field molecular orbital and configuration interaction (SCF-MOCI) calculations, 23 Solvents, magnesium... 

Assignment based on SCF-MO (self-consistent field molecular orbital) calculations. 

A question of philosophy arises concerning the molecular properties to be predicted by semiempirical treatments. On the one hand, the molecular orbital NDO methods were designed to mimic minimum basis set ab initio self-consistent field (SCF) property calculations parameters were chosen accordingly. Although internally consistent, this procedure is limited in accuracy by the minimum basis ab initio SCF results themselves. An alternative approach is to fit, and predia, measured physical properties. This is not internally consistent because experimental values cannot, in principle, be obtained from self-consistent field molecular orbital theory, regardless of basis set, because of the lack of elearon correlation. Furthermore, experimental values of properties obtained at room temperature cannot be equated to those calculated for 0° K. Nonetheless, the relatively high level of accuracy that can be achieved makes such an approach useful, and that is why it has been pursued by Dewar and co-workers in their series of M(odified)NDO methods. 

The use oE self-consistent field molecular orbital theory (SCF-MO) implies a number of approximations which are necessary, since the Schrodinger equation is generally unsolvable for three or more bodies. Thus SCF MO s represent an optimum approximation to the many-electron wave function. 

SCF-MO- Self-consistent field-molecular orbital-linear combination of atomic LCAO orbitals... 

Menna, Moccia and Randaccio 44), using the molecular-orbital approach in the one-centre, self-consistent-field approach (OCE—SCF— MO) developed by Moccia 45—47), reports the calculated vibrational frequencies of HF and HCl (using the FG-method (7)) ... 

Many problems in force field investigations arise from the calculation of Coulomb interactions with fixed charges, thereby neglecting possible mutual polarization. With that obvious drawback in mind, Ulrich Sternberg developed the COSMOS (Computer Simulation of Molecular Structures) force field [30], which extends a classical molecular mechanics force field by serai-empirical charge calculation based on bond polarization theory [31, 32]. This approach has the advantage that the atomic charges depend on the three-dimensional structure of the molecule. Parts of the functional form of COSMOS were taken from the PIMM force field of Lindner et al., which combines self-consistent field theory for r-orbitals ( nr-SCF) with molecular mechanics [33, 34]. 

---