Abstract
Non-covalent interactions pervade all matter and play a fundamental role in layered materials, biological systems, and large molecular complexes. Despite this, our accumulated understanding of non-covalent interactions to date has been mainly developed in the tens-of-atoms molecular regime. This falls considerably short of the scales at which we would like to understand energy trends, structural properties, and temperature dependencies in materials where non-covalent interactions have an appreciable role. However, as more reference information is obtained beyond moderately sized molecular systems, our understanding is improving and we stand to gain pertinent insights by tackling more complex systems, such as supramolecular complexes, molecular crystals, and other soft materials. In addition, accurate reference information is needed to provide the drive for extending the predictive power of more efficient workhorse methods, such as density functional approximations that also approximate van der Waals dispersion interactions. In this perspective, we discuss the first-principles approaches that have been used to obtain reference interaction energies for beyond modestly sized molecular complexes. The methods include quantum Monte Carlo, symmetry-adapted perturbation theory, non-canonical coupled cluster theory, and approaches based on the random-phase approximation. By considering the approximations that underpin each method, the most accurate theoretical references for supramolecular complexes and molecular crystals to date are ascertained. With these, we also assess a handful of widely used exchange-correlation functionals in density functional theory. The discussion culminates in a framework for putting into perspective the accuracy of high-level wavefunction-based methods and identifying future challenges.
Highlights
Across the natural sciences, intermolecular non-covalent interactions manifest in the properties and functions of all matter, from solid state materials to biological systems
A greater difficulty lies in approximating the long-range correlation energy, which arises from the Coulomb interaction between well-separated fluctuating charge distributions, broadly referred to as van der Waals (vdW) dispersion
Approaches based on symmetry adapted perturbation theory (SAPT)-density functional theory (DFT), CCSD(T), the random phase approximation (RPA), and diffusion Monte Carlo (DMC) have all been applied to the benzene crystal with varying success, as we presently discuss (Table III)
Summary
Intermolecular non-covalent interactions manifest in the properties and functions of all matter, from solid state materials to biological systems. The buckyball catcher complex (i.e., the host-guest complex shown in Fig. 2 and structure 4a shown in Fig. 3) is a prototypical example of a supramolecular complex, held together with dispersion interactions—predominantly π − π stacking In the experiment, such non-covalently bound systems are usually solvated, exhibiting similar intermolecular interactions as those present in much larger protein molecules and biological complexes. Taken together, provide strong evidence for the importance of higher-order (beyond second-order) interaction terms for the reliable and predictive description of intermolecular interactions in larger molecules in isolation and embedded in complex polarizable environments To this end, workhorse methods should be developed on the more complex testing ground of molecular crystals and supramolecular structures, which involve higher-order effects. III, we consider the key features of reference methods that have been used to compute larger molecular complexes
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