Berlin 2015 – scientific programme
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O: Fachverband Oberflächenphysik
O 85: Frontiers of Electronic Structure Theory: Many-Body Effects on the Nano-Scale VI
O 85.6: Talk
Thursday, March 19, 2015, 16:30–16:45, MA 004
Does GW obey the straight-line condition? — Matthias Dauth1,3, Fabio Caruso2, Stephan Kuemmel1, and •Patrick Rinke3 — 1University of Bayreuth, Germany — 2University of Oxford, England — 3Aalto University, Helsinki, Finland
Many-body theory in the GW approach has become the method of choice for calculating charged excitations in solids. Recently, it is also increasingly being applied to molecules, but fundamental questions regarding its accuracy remain. One such fundamental theorem requires that the total energy changes linearly with gradual (i.e. fractional) ionisation of the molecule. In this work we investigate, if GW is piecewise linear or if it exhibits a derivation of the straight line error (DSLE). Since the derivative of the total energy with respect to the electron number gives the ionisation energy, we quantify the DSLE by taking the difference between the energy of the highest occupied state of the neutral and the lowest unoccupied state of the singly ionised molecule (which would be equal in the DSLE-free case). We find for a subset of molecules from the quantum chemical G2 benchmark set, that the DSLE in self-consistent GW amounts to 1.1 eV on average. This DSLE can be mitigated in perturbative G0W0 by varying the starting point. We use density-functional theory as starting point and vary the amount of exact exchange α in the Perdew-Burke-Ernzerhof hybrid functional (PBEh). G0W0 becomes DSLE-free for α≈ 0.4. The average deviation from the experimental IPs is then very close to that of self-consistent GW and amounts to ∼0.25 eV.