Self-consistent field level

Unlike reactive diatomic chalcogen-nitrogen species NE (E = S, Se) (Section 5.2.1), the prototypical chalcogenonitrosyls HNE (E = S, Se) have not been characterized spectroscopically, although HNS has been trapped as a bridging ligand in the complex (HNS)Fc2(CO)6 (Section 7.4). Ab initio molecular orbital calculations at the self-consistent field level, with inclusion of electron correlation, reveal that HNS is ca. 23 kcal mof more stable than the isomer NSH. There is no low-lying barrier that would allow thermal isomerization of HNS to occur in preference to dissociation into H -1- NS. The most common form of HNS is the cyclic tetramer (HNS)4 (Section 6.2.1). 

B. Mennucci, R. Cammi and J. Tomasi, Excited states and solvatochromic shifts within a nonequilibrium solvation approach A new formulation of the integral equation formalism method at the self-consistent field, configuration interaction, and multiconfiguration self-consistent field level, J. Chem. Phys., 109 (1998) 2798. 

R. Cammi and J. Tomasi, Nonequilibrium solvation theory for the polarizable continuum model - a new formulation at the SCF level with application to the case of the frequency-dependent linear electric-response function, Int. J. Quantum Chem., (1995) 465-74 B. Mennucci, R. Cammi and J. Tomasi, Excited states and solvatochromic shifts within a nonequilibrium solvation approach A new formulation of the integral equation formalism method at the self-consistent field, configuration interaction, and multiconfiguration self-consistent field level, J. Chem. Phys., 109 (1998) 2798-807 R. Cammi, L. Frediani, B. Mennucci, J. Tomasi, K. Ruud and K. V. Mikkelsen, A second-order, quadratically... 

The generalised polarisability describing the response to an applied field of the electric field gradient at a nucleus in Br2 is calculated ab initio. A value of 110.55 Oq 1 is found at the self-consistent -field level. This is about half the value derived by modelling the measured nuclear quadrupole coupling constants of the ammonia-bromine complex. 

Most calculations available in the literature are concerned with the simplest alkyl azide and alkylnitrene, CH3-N3 (13) and CHg-N (14), respectively. The first ab initio theoretical study of the different electronic states of 14 was reported in 1974 by Yakony et al. The geometries of the lowest E, and electronic states of 14 were calculated at the self-consistent-field level of theory with a double basis set. The and states were predicted to lie 14,200 and 27,700 cm above the Ag ground state. Yakony et al. did not try to explain why 14 has been so difficult to isolate in the laboratory. ... 

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 . 

Results of quasirelativistic pseudopotential calculations at the unrestricted Hartree-Fock and multiconfiguration self-consistent field level for the equilibrium geometries of the lanthanide trihalides RX3 (X=F, Cl, Br, 1) have been presented by Cundari et al. (1995), Kobayashi et al. (1995) report Hartree-Fock and density-functional calculations of La2 Cgo. 

Most popular in the ab initio calculation of intermolecular potentials is the so-called supermolecule method, because it allows the use of standard computer programs for electronic structure calculations. This method automatically includes all the electrostatic, penetration and exchange effects. If the calculations are performed at the SCF (self-consistent field) level the induction effects are included, too, but the dispersion energy is not. The latter, which is an intermolecular electron correlation effect, can be obtained by configuration interaction (Cl), coupled cluster (CC) calculations or many-body perturbation theory (MBPT). These calculations are all plagued... 

For the MFP approach, most calculations have used SCF wavefunctions. However, very recently, Bak et al. have implemented the MFP theoretical approach at the correlated multiconfigurational self-consistent-field level by using the complete active space wavefunctions (CASSCF) expressed over conventional and gauge invariant basis sets. 

Let us give a few examples of how conformational transition states may be determined in an automated manner. The calculations have been carried out with HERMIT-SIRIUS-ABACUS [31, 32, 33] at the self-consistent field level using a minimal basis set. A simple example is provided by PH5, whose equilibrium structure is a trigonal bipyramid of D h symmetry. Starting at equilibrium and applying the TRIM method without symmetry constraints, we arrive after nine iterations at a square pyramidal transition state of C4V symmetry. This structure represents the barrier to a Berry pseudo-rotation connecting two equivalent Dsh equilibrium structures, as confirmed by walking down the other side of the barrier. 

Formulation of the Integral Equation Formalism Method at the Self-Consistent Field, Configuration Interaction, and Multiconfiguration Self-Consistent Field Level. 

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