Generalized valence bond calculations

Tel. 818-568-9392, fax 818-568-9778, e-mail info psgvb.com Pseudospectral Generalized Valence Bond calculations on molecules, clusters, and crystals. Cray, Silicon Graphics, Hewlett-Packard, IBM RS/6000, and Convex. The graphical user interface, PS-GUI, runs on Silicon Graphics, Hewlett-Packard, and Sun. 

N=N double bond character is indicated, as also supported by generalized valence bond calculations on the simple H2NN.394 Compound 335 dimerizes to the corresponding tetrazene on warming, concomitant with fragmentation via the diradical 356 (Eq. 105).395 Related five-membered 1,1-diazenes fragment to 1,4-diradicals in the gas phase.396... 

Hi) Three Q bonds (banana bonds). There are neither a nor n bonds, bnt the P-0 bond is a formal triple bond with the three curved regions of electron density between P and O in a symmetrical fashion. The remaining lone pair on the oxygen atom points away from phosphoras along the P-0 axis. The banana bond description originates from a Boys localization scheme [1,29-31] and from generalized valence bond calculations [6,32-34] for the phosphine oxide derivatives. 

The second isomer is the insertion product H-Sc-NKj". Generalized Valence Bond calculations on this system suggest that it is bound relative to Sc + NH3 by 16 kcal/mol. 

Of theoretical interest is the fact that generalized valence bond calculations in 1977 on vinylcarbene indicated that the planar singlet is 12-14 kcal/mol less stable... 

A number of types of calculations begin with a HF calculation and then correct for correlation. Some of these methods are Moller-Plesset perturbation theory (MPn, where n is the order of correction), the generalized valence bond (GVB) method, multi-conhgurational self-consistent held (MCSCF), conhgu-ration interaction (Cl), and coupled cluster theory (CC). As a group, these methods are referred to as correlated calculations. 

An MCSCF calculation in which all combinations of the active space orbitals are included is called a complete active space self-consistent held (CASSCF) calculation. This type of calculation is popular because it gives the maximum correlation in the valence region. The smallest MCSCF calculations are two-conhguration SCF (TCSCF) calculations. The generalized valence bond (GVB) method is a small MCSCF including a pair of orbitals for each molecular bond. 

A configuration interaction calculation uses molecular orbitals that have been optimized typically with a Hartree-Fock (FIF) calculation. Generalized valence bond (GVB) and multi-configuration self-consistent field (MCSCF) calculations can also be used as a starting point for a configuration interaction calculation. 

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. 

We discuss all of the key features of our current CASVB methodology for modem valence bond calculations on ground and excited states. The CASVB strategy may be used to generate compact representations of CASSCF wavefunctions or, alternatively, to perform the fully-variational optimization of various general types ofVB wavefunction. We report also a new application, namely to the fourteen % electrons of a planar dimethylenecyclobu-tadiene chain with three rings. 

Valence-bond calculations of 13C chemical shift parameters of alkanes permit the following general conclusions [204]. 

Another, quite independent theoretical assessment, based on ab initio calculations and the generalized valence bond method, found that the barrier height for cis,trans isomerization of cyclopropane is essentially the same (calculated value, 60.5 kcal mol-1) whether one or both of the thermal CH2 groups are rotated after opening of the CC bond 243. Thus, in 1972, there seemed to be general agreement among theoreticians that the stereomutations of cyclopropane should take place with k, about equal to kl2. 

Figure 6-22 Generalized valence-bond orbitals calculated for ethene by the ab initio method. The nuclei are located in the x,y plane of the coordinate system at the positions indicated by crosses. The long dashes correspond to locations of change of phase. The dotted lines are contour lines of electron amplitude of opposite phase to the solid lines. Top shows both m-bonding carbon orbitals (almost sp2), middle-left is the carbon orbital and middle-right the hydrogen orbital of one of the C-H bonds, and bottom represents a side view of the ir orbitals in perpendicular section to the x,y plane. (Drawings furnished by Dr. W, A. Goddard, III.)...

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