Ab Initio Molecular Dynamics is currently the most powerful computational technique that captures the dynamics of any physiochemical reacting system using only the knowledge of its elemental composition – hence ‘Ab Initio’. The basic workhorse -- the quantum density functional theory (Q-DFT), is used to compute the atomic forces via the Hellman Feynman theorem, which when neglecting the electron motion (assumed to be too fast on the time-scale of ion-motion – the Born Oppenheimer approximation), solves Newton’s equations of motion to obtain a time-dependent trajectory. This technique has lead to our understanding of a range of complex material systems, such as the interaction of water with oxide surfaces for fuel cell applications and chemical reactions of battery electrolytes on electrodes for Li-ion batteries, among other challenging systems studied at ORNL. The usefulness of the technique is accelerated by the availability of Q-DFT codes that scale very well on some of the world’s fastest supercomputers. Another class of quantum dynamics simulations introduces a time-dependent Hamiltonian for electrons, enabling description of (a) externally driven systems and molecules that are subject to a strong laser field for which a linear response approximation is not valid, (b) processes with coupling and energy transfer between nuclear and electronic degrees of freedom, and (c) non-equilibrium processes such as electron transfer in redox reactions.
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