Van der Waals (vdW) forces play an integral role in the binding and interaction energies of molecules in condensed phases; their long-range, many-body nature can modify phonons in molecular crystals and thereby impact thermodynamic stability. Typical macroscopic descriptions of such optical forces tend to ignore important atomistic effects arising at short (nanometric) scales, while microscopic treatments tend to ignore long-range, geometry-dependent electromagnetic effects. We describe an ab-initio approach to model such fluctuation-induced forces in mesoscopic systems comprising large molecules in the vicinity of macroscopic bodies, conjoining atomistic treatments of electronic and vibrational fluctuations derived from density functional theory in the former, with continuum descriptions of electromagnetic response in the latter, thereby accounting for many-body and multiple scattering effects to all orders. Such long-range electromagnetic effects become particularly important in situations where the finite sizes and shapes of the molecules and continuum bodies combine to create phonon polaritons with highly delocalized (nonlocal) charge distributions. We find that even in small molecules, but especially in elongated low-dimensional molecular systems, these effects modify van der Waals forces by orders of magnitude and produce qualitatively different behavior compared to predictions based on simple dipolar or pairwise approximations, valid only in atomically small or dilute molecular systems. In particular, we focus on the interactions of fullerenes, carbyne wires, and graphene sheets with one another and with a gold surface. We compare forces with and without phonon and at multiple temperatures, and compare our predictions to those obtained from commonly used dipolar and continuum treatments. In particular, we show that phonons can delocalize molecular charge distributions from a few angstroms to several nanometers, in ways that depend strongly on the shape of the molecules and their proximity to the surface. Even for small fullerenes, phonons can lead to force deviations at tens of nanometer separations from the surface compared to treatments lacking phononic effects, while for higher dimensional molecules such as elongated carbyne wires and graphene sheets, the nonlocality of these interactions produces nonmonotonic power laws that cannot be qualitatively captured by dipolar and/or continuum models.

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Alejandro W. Rodriguez, "Thermal van der Waals and optical forces among molecular and macroscopic bodies (Conference Presentation)," Proc. SPIE 10723, Optical Trapping and Optical Micromanipulation XV, 107230N (Presented at SPIE Nanoscience + Engineering: August 20, 2018; Published: 17 September 2018); https://doi.org/10.1117/12.2324045.5836012059001.