Dispersion interactions arise purely from long-range electronic correlation. Thus, in spite of being the weakest intermolecular force, they are present in all non-covalent dimers and are quantitatively significant in many cases. They dominate for example the interaction of non-polar systems, and play a significant role in complex extended assemblies such as proteins and DNA. Studies of short-range electron correlation have shown that it converges slowly with the basis set, revealing the difficulty in describing the electron-electron cusp. This understanding gave rise to explicitly correlated methods which improve the cusp description. Here, we investigate long-range electron correlation by examining the doubles amplitude tensor from coupled-cluster Singles and Doubles theory. We define and select a subset of amplitudes corresponding to dispersion interactions, and extract dispersion geminal functions from them through singular value decomposition. Our analysis reveals the differences between dispersion geminals and short-range correlated geminals. In addition, we identify the most important dispersion geminals, allowing us to write compact expansions of the dispersion energy with minimal loss of accuracy. The transferability of dispersion geminals for a single system interacting with different partners is also investigated. In addition to yielding qualitative insights into the description of long-range correlation, and in particular dispersion, this study offers a way to obtain a well-defined dispersion energy in the basis set limit, which is invaluable to energy decomposition schemes relying on Hilbert-space partitions.

Geminals for dispersion interactions and long-range correlation

253rd National Meeting (2017)

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