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Beyond Static Molecules: Modeling Quantum Fluctuations in Complex Molecular Environments

Periodic Reporting for period 3 - BeStMo (Beyond Static Molecules: Modeling Quantum Fluctuations in Complex Molecular Environments)

Reporting period: 2020-03-01 to 2021-08-31

We propose focused theory developments and applications, which aim to substantially advance our ability to model and understand the behavior of molecules in complex environments. From a large repertoire of possible environments, we have chosen to concentrate on experimentally-relevant situations, including molecular fluctuations in electric and optical fields, disordered molecular crystals, solvated (bio)molecules, and molecular interactions at/through low-dimensional nanostructures. A challenging aspect of modeling such realistic environments is that both molecular electronic and nuclear fluctuations have to be treated efficiently at a robust quantum-mechanical level of theory for systems with 1000s of atoms. In contrast, the current state of the art in the modeling of complex molecular systems typically consists of Newtonian molecular dynamics employing classical force fields. We will develop radically new approaches for electronic and nuclear fluctuations that unify concepts and merge techniques from quantum-mechanical many-body Hamiltonians, statistical mechanics, density-functional theory, and machine learning. Our developments will be benchmarked using experimental measurements with terahertz (THz) spectroscopy, atomic-force and scanning tunneling microscopy (AFM/STM), time-of-flight (TOF) measurements, and molecular interferometry.
Numerous fascinating and useful dynamical phenomena in complex molecular systems stem from an interplay between quantum-mechanical electronic and nuclear fluctuations. Many of the anomalies of water – the most ubiquitous liquid on Earth – are the result of a subtle coupling between its peculiar hydrogen bonded network and quantized nuclear fluctuations. Another particularly striking example is the aspirin molecular crystal, in which the prevalent form I polymorph is stabilized by entropy stemming from an unexpected dynamical coupling between non-covalent van der Waals interactions and quantized lattice vibrations. Additional prominent examples of coaction of electronic and nuclear quantum fluctuations include the peculiar ability of protons to permeate atomically-thin membranes at room temperature, the pervasive observation of delocalized boson peaks in disordered molecular materials, and non-trivial wavelike behavior of large molecules in electric and optical fields. Our ability to atomistically model and understand all of these important phenomena requires substantial breakthroughs.
Our final goal is to bridge the accuracy of quantum mechanics with the efficiency of force fields, enabling large-scale predictive quantum molecular dynamics simulations for complex systems containing 1000s of atoms, and leading to novel conceptual insights into quantum-mechanical fluctuations in large molecular systems. The project goes well beyond the presently possible applications and once successful will pave the road towards having a suite of first-principles-based modeling tools for a wide range of realistic materials, such as biomolecules, nanostructures, disordered solids, and organic/inorganic interfaces.
The BeStMo project had significant advances in all workpackages WP1, WP2, WP3, and WP4.
In WP1, we have developed an optimized parameterization of the Quantum Drude Oscillator (QDO) model for arbitrary atoms and molecules. This is a long-standing problem and our results exceed the expectations laid in the ERC proposal. We have published our initial results in Phys. Rev. Research and Nat. Comms. Currently, we are writing a paper for Phys. Rev. Lett. that presents our parameterization for all atoms in the periodic table. We have also developed a method to solve coupled QDOs subject to arbitrary electric fields, allowing us to obtain exact numerical solutions for any number of QDOs under arbitrary fields. This work will be published in J. Phys. Chem. Lett. and Phys. Rev. Research.
In WP3, we have pushed the applicability of our machine learning (ML) methods to realistic large molecules and extended solids. We have published several original manuscripts presenting the extension of our GMDL and SchNet models to large systems (published in JPCL, Nat. Comms. and Nat. Comms, in review). We have also published two comprehensive reviews on the quickly developing field of computational chemistry driven by ML (both in Chemical Reviews – the prime review journal of the American Chemical Society).
In WP4, we have applied our developed path integral methods to study nuclear quantum effects (NQE) for a wide range of molecules. We arrived at a surprising conclusion that NQEs often stabilize global minima of molecules, instead of the expected destabilization (paper published in Nat. Comms.).
Overall, I feel that, despite the COVID-related delay in our planned communication and dissemination activities, the original plan of the proposal has been fully accomplished and, indeed, exceeded in all workpackages.
The main goal of the BeStMo project is to develop pioneering methods that include both electronic and nuclear quantum-mechanical many-particle fluctuations in the modeling of dynamics of complex molecular systems with 1000s of atoms. By now, we have essentially accomplished this goal for large biomolecular systems by combining machine learning models for short-range interactions with electronic models based on quantum Drude oscillators (QDOs). The final goal of "push-button" simulations of systems with 1000s of atoms requires integrating several of the tools we developed into a consistent open-source framework. This will be the main goal for the remainder of the BeStMo project.