From owner-chemistry@ccl.net Sun Mar 17 15:33:01 2019
From: "Stefan Grimme grimme-*-thch.uni-bonn.de" <owner-chemistry]_[server.ccl.net>
To: CCL
Subject: CCL: comment on double hybrids and DFT functional popularity
Message-Id: <-53655-190317145827-20135-pxGZmF0KtZr43cKDWa0zYQ]_[server.ccl.net>
X-Original-From: "Stefan  Grimme" <grimme%%thch.uni-bonn.de>
Date: Sun, 17 Mar 2019 14:58:24 -0400


Sent to CCL by: "Stefan  Grimme" [grimme|,|thch.uni-bonn.de]
>(a) Double hybrids include a MP2-like perturbation energy as one of the >components of their energy functional. The MP2 energy terms include a >difference between occupied and virtual orbitals in their denominator. >Metallic conductors (e.g., solids like Cu, Ag, Au, etc.) have a partially >filled conduction band, so the energy difference between "occupied" and >"virtual" orbitals is zero. This makes the MP2 energy blow up for these >materials. Therefore, double hybrids are not suitable for computations on  >metallic conductors or other materials like graphene that have zero band-gap.

Dear all,
just a comment to the above statement which is incomplete: 
the virtual orbital dependent (non-local correlation) part in double-hybrids could in principle be computed with any WFT approach. In fact DHDFs using RPA correlation instead of MP2 exist (e.g. PWRB95, see Phys. Chem. Chem. Phys. (2016), 18, 20926-20937. DOI: 10.1039/c5cp06600j) and perform as
expected well in particular for metallic cases (even when applying computational efficient approximations, see J. Chem. Theory Comput. (2018), 14, 25962608. DOI: 10.1021/acs.jctc.7b01183). Such "non-standard" DHDFs don't suffer from the PT2/zero-gap/metallic problem and are may be as robust as RPA itself.
Best
Stefan