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Quantum nuclear effects plays a central role in determining properties of systems that contain light nuclei. For instance, the large deviation of the heat capacity of a solid from the Delong’s Petit limit and of the particle momentum distribution from the MaxwellBoltzmann behaviour are direct manifestations of the quantum nature of nuclei. While these effects can be accurately modelled in atomistic simulations by employing the imaginary time path integral (PI) technique [1], the high computational cost of running PI simulations has prevented their widespread use. In this talk, I will introduce molecular dynamics based methods [2, 3, 4, 5] that substantially reduce the computational cost of PI simulations and their implementation in an open source software iPI. [6] Going beyond benchmarks, I will demonstrate the relevance of these advances by studying different properties and classes of materials – such as the proton momentum distribution in water that relates to the local structure of protons in ice [4] and facilitates interpretation of complex Deep Inelastic Neutron scattering experiments, quantum effects that facilitate isotope separation in porous organic cages [7], tuning of thermal properties of metalorganic cages loaded with greenhouse gases [5], and quantitative estimation of quantum mechanical effects that stabilize pharmaceutically active molecular crystals [8] – at a fraction of the computational cost if using conventional techniques.
References

[1] David Chandler and Peter G. Wolynes. Exploiting the isomorphism between quantum theory and classical statistical mechanics of polyatomic fluids. The Journal of Chemical Physics, 74(7):4078– 4095, April 1981.

[2] V. Kapil, J. VandeVondele, and M. Ceriotti. Accurate molecular dynamics and nuclear quantum effects at low cost by multiple steps in real and imaginary time: Using density functional theory to accelerate wavefunction methods. The Journal of Chemical Physics, 144(5):054111, February 2016.

[3] Venkat Kapil, J ̈org Behler, and Michele Ceriotti. High order path integrals made easy. The Journal of Chemical Physics, 145(23):234103, December 2016.

[4] Venkat Kapil, Alice Cuzzocrea, and Michele Ceriotti. Anisotropy of the Proton Momentum Distribution in Water. The Journal of Physical Chemistry B, 122(22):6048–6054, June 2018.

[5] Venkat Kapil, Jelle Wieme, Steven Vandenbrande, Aran Lamaire, Veronique Van Speybroeck, and Michele Ceriotti. Modeling the Structural and Thermal Properties of Loaded Metal–Organic Frameworks. An Interplay of Quantum and Anharmonic Fluctuations. Journal of Chemical Theory and Computation, 15(5):3237–3249, May 2019.

[6] Venkat Kapil, Mariana Rossi, Ondrej Marsalek, Riccardo Petraglia, Yair Litman, Thomas Spura, Bingqing Cheng, Alice Cuzzocrea, Robert H. Meißner, David M. Wilkins, Benjamin A. Hel frecht, Przemyslaw Juda, S ́ebastien P. Bienvenue, Wei Fang, Jan Kessler, Igor Poltavsky, Steven Vandenbrande, Jelle Wieme, Clemence Corminboeuf, Thomas D. Ku ̈hne, David E. Manolopou los, Thomas E. Markland, Jeremy O. Richardson, Alexandre Tkatchenko, Gareth A. Tribello, Veronique Van Speybroeck, and Michele Ceriotti. iPI 2.0: A universal force engine for advanced molecular simulations. Computer Physics Communications, 236:214–223, March 2019.

[7] Ming Liu, Linda Zhang, Marc A. Little, Venkat Kapil, Michele Ceriotti, Siyuan Yang, Lifeng Ding, Daniel L. Holden, Rafael BalderasXicoht ́encatl, Donglin He, Rob Clowes, Samantha Y. Chong, Gisela Schu ̈tz, Linjiang Chen, Michael Hirscher, and Andrew I. Cooper. Barely porous organic cages for hydrogen isotope separation. Science, 366(6465):613–620, November 2019.

[8] Venkat Kapil, Edgar Engel, Mariana Rossi, and Michele Ceriotti. Assessment of Approxi mate Methods for Anharmonic Free Energies. Journal of Chemical Theory and Computation, 15(11):5845–5857, November 2019.