Abstract
A multiple-timestep ab initio molecular dynamics scheme based on varying the two-electron integral screening method used in Hartree-Fock or density functional theory calculations is presented. Although screening is motivated by numerical considerations, it is also related to separations in the length- and timescales characterizing forces in a molecular system: Loose thresholds are sufficient to describe fast motions over short distances, while tight thresholds may be employed for larger length scales and longer times, leading to a practical acceleration of ab initio molecular dynamics simulations. Standard screening approaches can lead, however, to significant discontinuities in (and inconsistencies between) the energy and gradient when the screening threshold is loose, making them inappropriate for use in dynamics. To remedy this problem, a consistent window-screening method that smooths these discontinuities is devised. Further algorithmic improvements reuse electronic-structure information within the dynamics step and enhance efficiency relative to a naı̈ve multiple-timestepping protocol. The resulting scheme is shown to realize meaningful reductions in the cost of Hartree-Fock and B3LYP simulations of a moderately large system, the protonated sarcosine/glycine dipeptide embedded in a 19-water cluster.
Highlights
Ab initio molecular dynamics (AIMD) techniques combine the locality and efficiency of classical nuclear dynamics with on-the-fly generation of forces from electronic structure theory to generate real-time chemical information at a microscopic level.1–4 Electronic structure theory can provide significantly more accurate forces than classical force fields, for systems exhibiting strong polarization, charge transfer, and bond rearrangements
The MTS scheme — as implemented in a development version of Q-Chem34 — is applied to a biological model, the protonated sarcosine-glycine dipeptide embedded in a 19-water cluster. This choice of model was inspired by recent experiments using gas-phase SarGlyH+ as a testbed for understanding the structural and dynamical effects of peptide methylation,60,61 these phenomena are not addressed in this work
This work addressed the question of whether multiple-timestep methods could be applied to Hartree–Fock or DFT molecular dynamics on a firmer-than-ad-hoc basis
Summary
Ab initio molecular dynamics (AIMD) techniques combine the locality and efficiency of classical nuclear dynamics with on-the-fly generation of forces from electronic structure theory to generate real-time chemical information at a microscopic level. Electronic structure theory can provide significantly more accurate forces than classical force fields, for systems exhibiting strong polarization, charge transfer, and bond rearrangements. Ab initio molecular dynamics (AIMD) techniques combine the locality and efficiency of classical nuclear dynamics with on-the-fly generation of forces from electronic structure theory to generate real-time chemical information at a microscopic level.. Electronic structure theory can provide significantly more accurate forces than classical force fields, for systems exhibiting strong polarization, charge transfer, and bond rearrangements. The cost of electronic-structure forces is many orders of magnitude larger than that of force fields, effectively prohibiting the use of AIMD for many systems and timescales of chemical interest. Non-polarizable force fields partition the potential energy of chemical systems into several bonded and non-bonded contributions,. While the Lennard-Jones potentials typically used to represent the van der Waals dispersion forces are basically short-range (decaying as r−6), the (1/r) Coulomb potential mediating the electrostatics is inherently long-range. Evaluation of the electrostatic energy and forces is the most timeconsuming part of force-field treatments of large systems.
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