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ASES Report Summary

Project ID: 320723
Funded under: FP7-IDEAS-ERC
Country: Germany

Final Report Summary - ASES (Advancing computational chemistry with new accurate, robust and scalable electronic structure methods)

The objective of the current work was to develop accurate, robust and scalable theoretical methods for treating the electron correlation problem in large molecules. "Scalable" means that the computational effort should scale linearly with molecular size, and that the elapsed time scales inversely linear with the number of processors or computing cores. In the ideal case that both conditions are fulfilled, the size of molecules that can be treated in a fixed time grows linearly with the number of compute cores. Linear scaling in electron correlation calculations can be achieved by localizing the occupied molecular orbitals, and using suitable local virtual orbitals, such as projected atomic orbitals (PAOs), orbital specific virtuals (OSVs), or pair natural orbitals (PNOs). This allows to restrict the correlation space for each electron pair to a local domain of virtual orbitals, and to treat the correlation of distant electrons using simplified approximations. The basis set incompleteness error can be much reduced by including explicitly correlated (F12) terms in the wave function, and in our work this was combined with local approximations to yield overall linear scaling. The F12 methods yield results close to the complete basis set limit with rather small (double or triple zeta) basis sets. Furthermore, the F12 terms also reduce errors due to the local domain approximations. We have implemented from scratch a new generation of explicitly correlated local correlation methods, which are well parallelized and can be applied to molecules with hundreds of atoms. We have extensively benchmarked the accuracy of these methods for absolute and relative correlation energies, and showed that the errors introduced by local approximations in energy differences such as reaction or activation energies are within 1 kcal/mol, i.e. well within chemical accuracy. Local methods have been developed for treating the dynamical correlation in so-called single-reference cases, as well as for systems with strong non-dynamical correlation effects, which are for example often present in transition metal clusters. Furthermore, we have developed explicitly correlated coupled-cluster methods not only for computing the energy, but also for analytical energy gradients (forces) and molecular response properties. The methods will be made available for general use in the Molpro package of ab initio programs.

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