FDE-vdW: A van der Waals Inclusive Subsystem Density-Functional Theory

TL;DR

FDE-vdW is a new, formally exact van der Waals inclusive subsystem DFT method that accurately computes weak interactions by incorporating long-range correlation effects through a non-additive correlation functional.

Contribution

It introduces a novel, exact formulation of van der Waals interactions within subsystem DFT, with practical schemes and demonstrated improvements over previous methods.

Findings

FDE-vdW accurately predicts binding energies of weakly bound complexes.

The method is Casimir-Polder consistent at large separations.

FDE-vdW shows significant improvement over semilocal DFT approaches.

Abstract

We present a formally exact van der Waals inclusive electronic structure theory, called FDE-vdW, based on the Frozen Density Embedding formulation of subsystem Density-Functional Theory. In subsystem DFT, the energy functional is composed of subsystem additive and non-additive terms. We show that an appropriate definition of the long-range correlation energy is given by the value of the non-additive correlation functional. This functional is evaluated using the Fluctuation-Dissipation Theorem aided by a formally exact decomposition of the response functions into subsystem contributions. FDE-vdW is derived in detail and several approximate schemes are proposed, which lead to practical implementations of the method. We show that FDE-vdW is Casimir-Polder consistent, i.e. it reduces to the generalized Casimir-Polder formula for asymptotic inter-subsystems separations. Pilot calculations of binding energies of 13 weakly bound complexes singled out from the S22 set show a dramatic improvement upon semilocal subsystem DFT, provided that an appropriate exchange functional is employed. The convergence of FDE-vdW with basis set size is discussed, as well as its dependence on the choice of associated density functional approximant.