Range separated hybrids of pair coupled cluster doubles and density functionals

Abstract

Pair coupled cluster doubles (pCCD) is a size-consistent, size-extensive, low-cost simplification of CCD that has been shown to be able to describe static correlation without breaking symmetry. We combine pCCD with Kohn-Sham functionals of the density and the local pair density in order to incorporate dynamic correlation in pCCD while maintaining its low cost. Double counting is eliminated by splitting the (interelectron) Coulomb operator into complementary short- and long-range parts, and evaluating the two-body energy with pCCD in the long-range and with density functionals in the short-range. This simultaneously suppresses self-interaction in the Hartree-exchange term of the functionals. Generalizations including a fraction of wavefunction two-body energy in the short-range are also derived and studied. The improvement of our pCCD+DFT hybrids over pCCD is demonstrated in calculations on benchmarks where both types of correlation are important.

Benchmark of Dynamic Electron Correlation Models for Seniority-Zero Wave Functions and Their Application to Thermochemistry

“…This can be achieved a posteriori using PT approaches, [46,47] coupled cluster corrections, [50,51] or DFT-type methods. [54,55] In this work, we have extended the previously presented PT models with an AP1roG reference function and benchmarked those models against spectroscopic constants for multiply bonded diatomics and thermochemical data extrapolated to the basis set limit. Most importantly, combining AP1roG with the investigated corrections allows us to reliably model molecular systems dominated by both static/nondynamic and dynamic electron correlation.…”

“…A different, computationally feasible approach suitable for strongly-correlated systems uses seniority-zero wavefunctions to describe the static/nondynamic part of the electron correlation en-ergy. [31][32][33][34][35][36][37][38][39][40][41][42][43][44] The missing dynamic electron correlation effects are included a posteriori in these ansätze using, for instance, many-body perturbation theory [45][46][47], coupled-cluster theory [48][49][50][51][52], extended random phase approximation [53], and density functional theory (DFT) corrections [54,55].…”

Performance of a nonempirical exchange functional from density matrix expansion: comparative study with different correlations

“…Each type of local ingredients not only enables better treatment of the one-electron region but also liberates the functional form so that more exact constraints can be satisfied. Inclusion of nonlocal information, such as the exact exchange energy density, leads to nonlocal functionals, 10,[35][36][37][38][39][40][41] which are more complicated in form and more expensive in computational cost, but can be highly accurate for the description of band gaps, 39,42 charge transfer, reaction barriers, and so forth.…”

Global Hybrid Multiconfiguration Pair-Density Functional Theory

“…In addition, the MCPDFT formalism could be futher improved by generalizing the OTPD functionals to consider range separation of the coulomb interaction (see Ref. 22, for example) or double hybrization.…”