We developed the cluster-in-molecule (CIM) local correlation approach with the periodic boundary condition (PBC-CIM), for the electron-correlation calculations of various crystals. This approach makes the molecular electronic-correlation methods available to crystals, including MP2, CCSD and the much powerful DLPNO-CCSD(T). In this approach, the correlation energy per unit cell of a periodic system can be evaluated as the summation of the contributions from a series of finite-sized clusters, which is defined as a set of mutually neighboring occupied orbirals and virtual orbitals. The electron-correlation calculations are performed independently on these clusters, thus the highly expensive calculation of the whole periodic system is avoided. We also proposed an efficient distant-pair correction, which made a tremendous improvement to the accuracy of the CIM approach. We emphaise that the basis set's effect can be very significant in the electron-correlation calculation for some problems, such as the cohesive energies and the polymorphism of molecular crystals. We developed the CIM approach with the F12 explicit correlation method to diminish the error from basis sets incompleteness (BSIE), in hopes of handling these problems in the future.

Cluster in molecule local correlation methods for periodic systems

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