Topological analysis of electron momentum densities and the bond directional principle: the first-row hydrides, AH, and homonuclear diatomic molecules, A2

1996 ◽  
Vol 74 (6) ◽  
pp. 1187-1191 ◽  
Author(s):  
Jiahu Wang ◽  
B. James Clark ◽  
Hartmut Schmider ◽  
Vedene H. Smith
Keyword(s):  
Diatomic Molecules ◽  
Topological Analysis ◽  
Momentum Density ◽  
Wave Functions ◽  
Basis Set ◽  
Electron Momentum ◽  
Hartree Fock ◽  
Topological Features ◽  
Homonuclear Diatomic Molecules ◽  
Slater Basis

Topological analysis of electron momentum densities of the first-row hydrides and homonuclear diatomic molecules has been carried out. The densities and their curvatures were calculated from wave functions of near Hartree–Fock quality using a Slater basis. The bond directional principle has been discussed through the topological properties of electron momentum densities. Basis set effects on the topological features have also been addressed. Key words: electron momentum density, the bond directional principle, topological analysis, first-row hydrides, homonuclear diatomic molecules.

Basis-Set Expansions of Relativistic Electronic Wave Functions

2007 ◽  
Author(s):  
Kenneth G. Dyall ◽  
Knut Faegri
Keyword(s):  
Relativistic Quantum ◽  
Finite Basis ◽  
Wave Functions ◽  
Basis Set ◽  
Electronic Wave ◽  
Nonrelativistic Case ◽  
Hartree Fock ◽  
Electronic Wave Functions ◽  
Basis Set Expansion ◽  
Molecular Calculations

There have been several successful applications of the Dirac–Hartree–Fock (DHF) equations to the calculation of numerical electronic wave functions for diatomic molecules (Laaksonen and Grant 1984a, 1984b, Sundholm 1988, 1994, Kullie et al. 1999). However, the use of numerical techniques in relativistic molecular calculations encounters the same difficulties as in the nonrelativistic case, and to proceed to general applications beyond simple diatomic and linear molecules it is necessary to resort to an analytic approximation using a basis set expansion of the wave function. The techniques for such calculations may to a large extent be based on the methods developed for nonrelativistic calculations, but it turns out that the transfer of these methods to the relativistic case requires special considerations. These considerations, as well as the development of the finite basis versions of both the Dirac and DHF equations, form the subject of the present chapter. In particular, in the early days of relativistic quantum chemistry, attempts to solve the DHF equations in a basis set expansion sometimes led to unexpected results. One of the problems was that some calculations did not tend to the correct nonrelativistic limit. Subsequent investigations revealed that this was caused by inconsistencies in the choice of basis set for the small-component space, and some basic principles of basisset selection for relativistic calculations were established. The variational stability of the DHF equations in a finite basis has also been a subject of debate. As we show in this chapter, it is possible to establish lower variational bounds, thus ensuring that the iterative solution of the DHF equations does not collapse. There are two basically different strategies that may be followed when developing a finite basis formulation for relativistic molecular calculations. One possibility is to expand the large and small components of the 4-spinor in a basis of 2-spinors. The alternative is to expand each of the scalar components of the 4-spinor in a scalar basis. Both approaches have their advantages and disadvantages, though the latter approach is obviously the easier one for adapting nonrelativistic methods, which work in real scalar arithmetic.

scholarly journals An analysis of SCF and geometry convergence for diatomic molecules

2011 ◽  
Vol 9 (4) ◽  
pp. 567-571 ◽  
Author(s):  
Cory Pye
Keyword(s):  
Diatomic Molecules ◽  
Basis Set ◽  
Convergence Behavior ◽  
Hartree Fock ◽  
Convergence Study

AbstractA geometry and SCF convergence study of Hartree-Fock calculations using the 6-31G* basis set is carried out on the set of all possible diatomic molecules formed from atoms with Z≤36. The utility of Hartree-Fock calculations using the smaller STO-3G basis set to improve the convergence behavior is demonstrated.

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