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# 5.11.1 Overview

(February 4, 2022)

From the perspective of perturbation theory, Chai and Chen181 proposed a systematic procedure for the evaluation of the derivative discontinuity of the exchange-correlation energy functional in Kohn-Sham (KS) DFT, wherein the exact derivative discontinuity can in principle be obtained by summing up all the perturbation corrections to infinite order. Truncation of the perturbation series at low order yields an efficient scheme for obtaining the approximate derivative discontinuity. In particular, the first-order correction term is equivalent to the frozen-orbital approximation method. Its implementation in Q-Chem supports only local and GGA functionals at present, not meta-GGA, hybrid, or non-local functionals. Job control variables and examples appear below.

FOA_FUNDGAP

FOA_FUNDGAP
Compute the frozen-orbital approximation of the fundamental gap.
TYPE:
Boolean
DEFAULT:
FALSE
OPTIONS:
FALSE Do not compute FOA derivative discontinuity and fundamental gap. TRUE Compute and print FOA fundamental gap information. Implies KS_GAP_PRINT.
RECOMMENDATION:
Use in conjunction with KS_GAP_UNIT if true.

KS_GAP_PRINT

KS_GAP_PRINT
Control printing of (generalized Kohn-Sham) HOMO-LUMO gap information.
TYPE:
Boolean
DEFAULT:
false
OPTIONS:
false (default) do not print gap information true print gap information
RECOMMENDATION:
Use in conjunction with KS_GAP_UNIT if true.

KS_GAP_UNIT

KS_GAP_UNIT
Unit for KS_GAP_PRINT and FOA_FUNDGAP (see Section 5.11)
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
0 (default) hartrees 1 eV
RECOMMENDATION:
none

Example 5.23  Frozen-orbital approximation of derivative discontinuity with PBE and LFAs-PBE functionals on carbon atom.

$comment Frozen-orbital derivative discontinuity, C atom, PBE$end

$molecule 0 3 C$end

$rem BASIS 6-31G* METHOD PBE FOA_FUNDGAP true KS_GAP_UNIT 1 ! print gap info in eV THRESH 14$end

@@@

$comment with LFAs-PBE functional instead$end

$molecule READ$end

$rem BASIS 6-31G* SCF_GUESS READ EXCHANGE gen FOA_FUNDGAP true KS_GAP_UNIT 1 THRESH 14$end

$xc_functional X PBE 1.0 X LFAs 1.0 C PBE 1.0$end


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