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PubMed2005

Density functional energetics of alpha-quartz for calibration of SiO2 interatomic potentials.

All-electron, Gaussian basis density functional theory (DFT) total energies for alpha-quartz were generated on a substantial grid of points for eventual use in a first-principles check of calibration of a widely used effective interionic potential for SiO2. Unlike a previous all-electron, Gaussian basis calculation, we do not find a weak double minimum with respect to cell volume. Rather there is a weak double minimum with respect to internal cell parameters that is at the margins of numerical precision and, hence, may only be the signal of a very flat potential surface. The results show typical dependence on the choice of approximate DFT functional and differ from experimental values enough that both aspects may be problematic for parametrization of potentials.

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PubMed2004

A natural linear scaling coupled-cluster method.

It is shown that using an appropriate localized molecular orbital (LMO) basis, one is able to calculate coupled-cluster singles and doubles (CCSD) wave functions and energies for very large systems by performing full CCSD calculations on small subunits only. This leads to a natural linear scaling coupled-cluster method (NLSCC), in which total correlation energies of extended systems are evaluated as the sum of correlation energy contributions from individual small subunits within that system. This is achieved by defining local occupied orbital correlation energies. These are quantities, which in the LMO basis become transferable between similar molecular fragments. Conventional small scale existing molecular CCSD codes are all that is needed, the local correlation effect being simply transmitted via the appropriate LMO basis. Linear scaling of electronic correlation energy calculations is thus naturally achieved using the NLSCC approach, which in principle can treat nonperiodic extended systems of infinite basis set size. Results are shown for alkanes and several polyglycine molecules and the latter compared to recent results obtained via an explicit large scale LCCSD calculation. (c) 2004 American Institute of Physics.

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