DIELECTRIC FUNCTION FOR SI BY A LINEAR COMBINATION OF ATOMIC ORBITALS METHOD

We calculate the frequency- and wave-vector-dependent dielectric functions for Si in the random-phase approximation. A first-principles pseudopotential method is employed, and the wave functions are expanded in terms of atomic-like Gaussian orbitals. All the matrix elements of the dielectric function are evaluated in an analytical fashion in real space. The results of our calculations are in good agreement with those calculated with a plane-wave basis set. In the optical limit q-->0, the plasmon excitation is damped significantly due to particle-hole excitation, and its peak is found to have a large width of about 2.2 eV. For the wave-vectors considered along the [001] direction, the plasmon peaks show less dispersive behavior compared with those of simple metals. Testing sum rules, we find the importance of the local field effect in Si.