The extended CC2 model ECC2

Abstract

We present a size-extensive extension to the CC2 model that avoids the complications with quasi-degeneracies that are present in the CC2 model and related perturbation theory-based approaches. The formulation also provides a consistent model for treating different parts of a molecular system at different levels of electron correlation. Such a subsystem approach leads to large reductions in the computational requirements without compromising the accuracy. In this initial study, we focus on static molecular properties.

Keywords:

• ECC2
• coupled cluster
• electron correlation
• subsystems

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Corrigendum

Acknowledgements

We acknowledge support from the Spanish MINECO through project number CTQ2010-19738 and NOTUR for computer time.

Notes

^ao – number of occupied orbitals; v – number of virtual orbitals.

^ao – number of occupied orbitals; v – number of virtual orbitals.

^ao – number of occupied orbitals; v – number of virtual orbitals.

^a The hydrogen atom being abstracted.