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Person# Martin Peter Bircher

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Density functional theory

Density-functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (

Basis set (chemistry)

In theoretical and computational chemistry, a basis set is a set of functions (called basis functions) that is used to represent the electronic wave function in the Hartree–Fock method or density-fu

Molecular dynamics

Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giv

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Martin Peter Bircher, Ursula Röthlisberger, Justin Villard

The calculation of electron correlation is vital for the description of atomistic phenomena in physics, chemistry, and biology. However, accurate wavefunction-based methods exhibit steep scaling and often sluggish convergence with respect to the basis set at hand. Because of their delocalization and ease of extrapolation to the basis-set limit, plane waves would be ideally suited for the calculation of basis-set limit correlation energies. However, the routine use of correlated wavefunction approaches in a plane-wave basis set is hampered by prohibitive scaling due to a large number of virtual continuum states and has not been feasible for all but the smallest systems, even if substantial computational resources are available and methods with comparably beneficial scaling, such as the Moller-Plesset perturbation theory to second order (MP2), are used. Here, we introduce a stochastic sampling of the MP2 integrand based on Monte Carlo summation over continuum orbitals, which allows for speedups of up to a factor of 1000. Given a fixed number of sampling points, the resulting algorithm is dominated by a flat scaling of similar to O(N-2). Absolute correlation energies are accurate to

Martin Peter Bircher, Ursula Röthlisberger

Exact exchange is a primordial ingredient in Kohn–Sham Density Functional Theory based Molecular Dynamics (MD) simulations whenever thermodynamic properties, kinetics, barrier heights or excitation energies have to be predicted with high accuracy. However, the cost of such calculations is often prohibitive, restricting the use of first principles MD to (semi-)local density functionals, in particular in a plane wave basis. We have recently proposed the use of coordinate-scaled orbitals to reduce the cost of the most expensive Fourier transforms during the calculation of the exact exchange potential of isolated systems. Here, we present the implementation and parallelisation of this coordinate scaling approach in the CPMD code and analyse its performance under different parallelisation schemes. We show that speedups increase with system size and that with an optimal configuration, speedups of up to one order of magnitude are possible with respect to conventional calculations. Simulations that have previously taken one week can therefore be finished within less than a day.

2020