Scheme for adding electron-nucleus cusps to Gaussian orbitals

A simple scheme is described for introducing the correct cusps at nuclei into orbitals obtained from Gaussian basis set electronic structure calculations. The scheme is tested with all-electron variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC) methods for the Ne atom, the H2 molecule, and 55 molecules from a standard benchmark set. It greatly reduces the variance of the local energy in all cases and slightly improves the variational energy. This scheme yields a general improvement in the efficiency of all-electron VMC and DMC calculations using Gaussian basis sets.     

A theory of the origin of silicon carbide polytypes

A new theory of the origin of SiC polytypes is presented in which the observed polytypes are stabilised as equilibrium phases at the high temperatures of growth, the structures becoming frozen in on cooling. Results of first principles pseudopotential total energy calculations show that the SiC polytypes containing only bands of width 2 and 3 are nearly degenerate in energy while all other polytypes have higher energies. We develop an inter-layer interaction model to describe the energies of the polytypes. Calculations of the phonon free energies of several SiC polytypes using the valence overlap shell model are described. These calculations show that the phonon free energy gives rise to effectively long ranged interactions between the SiC layers which can stabilise a large number of ordered polytypes as equilibrium phases.     

FTIR Emission Spectra, Molecular Constants, and Potential Curve of Ground State GeO

Extensive new high-temperature, high-resolution FTIR emission spectroscopy measurements for the five common isotopomers of GeO are combined with previous diode laser and microwave measurements in combined isotopomer analyses. New Dunham expansion parameters and an accurate analytical potential energy function are determined for the ground X1Sigma+ state. Copyright 1999 Academic Press.     

Quantum Monte Carlo study of the optical and diffusive properties of the vacancy defect in diamond

Fixed-node diffusion quantum Monte Carlo (DMC) calculations of the ground and excited state energetics of the neutral vacancy defect in diamond are reported. The multiplet structure of the defect is modeled using guiding wave functions of the Slater-Jastrow type with symmetrized multideterminant Slater parts. For the ground state we obtain the 1E state in agreement with experiment. The calculated energy of the lowest dipole allowed transition is consistent with the experimentally observed GR1 band, which has long been identified with the neutral vacancy in diamond, although no previous first-principles ab initio calculation of this transition exists. The calculated multiplet splitting of over 2 eV indicates the importance of a proper treatment of electron exchange and correlation in this system. DMC calculations of the formation and migration energy of the vacancy defect are presented.