Tensor-factorization for higher excitation order Coupled-Cluster amplitudes for calculating ground-state energy. Automatic code generation

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We will develop a new approach to electronic structure theory based on tensor-factorization of coupled-cluster amplitudes. Coupled-cluster methods have become a standard tool for ab-initio calculations of energies and properties of molecular systems. But even with efficient local implementations it is hardly possible to calculate large molecules beyond the level of coupled-cluster theory with singles, doubles and a perturbative triples correction (CCSD(T)) for ground-state energies. The main problems are the large amount of memory needed to store ground-state amplitudes in iterative procedures (in local formulations even the perturbative triples have to be calculated iteratively) and high effort needed for efficient implementation of these methods. In this project a new tensor-factorization technique will be applied on the coupled-cluster amplitudes beyond CCSD(T) and tested for accuracy and performance. The number of resulting amplitudes will be rather small and this quantities are then much easier to handle. An automatic code generator is essential for efficient and fast implementation of these methods and will be a part of this project. The new methods will make it possible to calculate highly accurate ground-state energies, which are needed e.g. in thermochemistry and studies of chemical reactivity.