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Valence charge density from many-particle calculations |
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The valence charge density is one of the most important properties of
each material. Most of the chemical behaviour is determined through
it. It is related to a variety of modells in chemistry to characterize the
kind of the chemical bond between the constituents of a material. Here a study of the valence charge distribution in GaAs is presented. The results are obtained from variational Quantum Monte Carlo (VMC) calculations with nonlocal pseudopotentials. Because the resulting many-body wave function is explicitly known the charge density is evaluated directly as the expectation value of Dirac's delta function. Our results are: (1) the density as a local observable is accessible by VMC with good statistics, (2) the bond in GaAs contains 0.1 electron more than in the superposition of pure atomic charges.
In this contour plot the valence charge density (a.u.) of GaAs in the (110) plane is displayed. The shadowed circles with As or Ga at the centre denote the area inside the pseudoradii where the resulting density is not equal to the true physical density (the price for using pseudopotentials). Outside the pseudoradii the density represents the true physical density and here we expect the main effect of chemical binding to take place. In fact, one can see that along the bond directions the density is slightly higher whereas off those directions the charge is rather isotropic. To obtain quantitative results the solid is divided into voronoi polyeders with the centers situated at the invariant lattice sites of the underlying diamond structure. The distribution of electrons over these volumes yields with sufficient statistics the rather small 0.1 electron binding charge value.
Just for additional illustration the density of GaAs is plotted along the (110) plane in threedimensional mode. The deep holes are the sites of the As and Ga core. Note how much the density is concentrated on As opposite to Ga. There are 5.1 of 8 electrons closer to As than to Ga, which is far from the usual notion of homeopolar or ionic.
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1997
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