  The cluster-in-molecule (CIM) local correlation approach with an accurate
distant pair correlation energy correction is presented. For large systems, the
inclusion of distant pair correlation energies is essential for the accurate
predictions of absolute correlation energies and relative energies. Here we
propose a simple and efficient scheme for evaluating the distant pair
correlation energy correction. The corrections can be readily extracted from
electron correlation calculations of clusters with almost no additional effort.
Benchmark calculations show that the improved CIM approach can recover more
than 99.97% of the conventional correlation energy. By combining the CIM
approach with the domain based local pair natural orbital (DLPNO) local
correlation approach, we have provided accurate binding energies at the
CIM-DLPNO-CCSD(T) level for a test set consisting of eight weakly bound
complexes ranging in size from 200 to 1027 atoms. With these results as the
reference data, the accuracy and applicability of other electron correlation
methods and a few density functional methods for large systems have been
assessed.
