Exact exchange


Inclusion of the non-local Fock operator as an exchange-correclation functional is an experimental feature in gpaw.

The current implementation lacks the following features:

  • Support for periodic systems. Actually, the code won’t complain, but the results have not been tested.
  • Support for k-point sampling. No consideration has been made as to multiple k-points, or even comlplex wave functions, so this definitely won’t work.
  • Forces. Force evaluations when including (a fraction of -) the fock operator in the xc-functional has been implemented, but has not been tested.
  • Speed. Inclusion of Fock exchange is exceedingly slow. The bottleneck is solving the poisson integrals of the Fock operator, which is currently done using an iterative real-space solver with a zero initial guess for the potential at each SCF cycle. This should be optimized.

One way to speed up an exact-exchange (or hybrid) calculation is to use the coarse grid (used for wave functions) instead of the finegrid (used for for densities) for the Fock potentials. This should give a speed-up factor of 8. This can be specified in the xc keyword like in this example coarse.py

Parallelization using domain decomposition is fully supported.

The Fock operator can be used to do the hybrid functional PBE0, and of course, Hartree-Fock type EXX. These are accessed by setting the xc keyword to PBE0 or EXX respectively.

The following functionals are suppported:

Functional Type Reference
EXX Global  
PBE0 Global [AB98]
B3LYP Global [Ba94]

Here “Global” denotes global hybrids, which use the same percentage of Hartree-Fock exchange for every point in space, while “RSF-SR” and “RSF-LR” denotes range-separated functionals which mix the fraction of Hartree-Fock and DFT exchange based on the spatial distance between two points, where for a “RSF-SR” the amount of Hartree-Fock exchange decrease with the distance and increase for a “RSF-LR”. See Range separated functionals (RSF) for more detailed information on RSF(-LR).

A thesis on the implementation of EXX in the PAW framework, and the specifics of the GPAW project can be seen on the literature page.

A comparison of the atomization energies of the g2-1 test-set calculated in VASP, Gaussian03, and GPAW is shown in the below two figures for the PBE and the PBE0 functional respectively.

../../_images/g2test_pbe.png ../../_images/g2test_pbe0.png

In the last figure, the curve marked GPAW (nonself.) is a non- selfconsistent PBE0 calculation using self-consistent PBE orbitals.

It should be noted, that the implementation lacks an optimized effective potential. Therefore the unoccupied states utilizing EXX as implemented in GPAW usually approximate (excited) electron affinities. Therefore calculations utilizing Hartree-Fock exchange are usually a bad basis for the calculation of optical excitations by lrTDDFT. As a remedy, the improved virtual orbitals (IVOs, [HA71]) were implemented. The requested excitation basis can be chosen by the keyword excitation and the state by excited where the state is counted from the HOMO downwards:

"""Calculate the excitation energy of NaCl by an RSF using IVOs."""
from ase.build import molecule
from ase.units import Hartree
from gpaw import GPAW, setup_paths
from gpaw.mpi import world
from gpaw.occupations import FermiDirac
from gpaw.test import equal, gen
from gpaw.eigensolvers import RMMDIIS
from gpaw.cluster import Cluster
from gpaw.lrtddft import LrTDDFT

h = 0.3  # Gridspacing
e_singlet = 4.3
e_singlet_lr = 4.3

if setup_paths[0] != '.':
    setup_paths.insert(0, '.')

gen('Na', xcname='PBE', scalarrel=True, exx=True, yukawa_gamma=0.40)
gen('Cl', xcname='PBE', scalarrel=True, exx=True, yukawa_gamma=0.40)

c = {'energy': 0.005, 'eigenstates': 1e-2, 'density': 1e-2}
mol = Cluster(molecule('NaCl'))
mol.minimal_box(5.0, h=h)
calc = GPAW(txt='NaCl.txt', xc='LCY-PBE:omega=0.40:excitation=singlet',
            eigensolver=RMMDIIS(), h=h, occupations=FermiDirac(width=0.0),
            spinpol=False, convergence=c)
(eps_homo, eps_lumo) = calc.get_homo_lumo()
e_ex = eps_lumo - eps_homo
equal(e_singlet, e_ex, 0.15)

lr = LrTDDFT(calc, txt='LCY_TDDFT_NaCl.log', istart=6, jend=7, nspins=2)
if world.rank == 0:
    lr2 = LrTDDFT('LCY_TDDFT_NaCl.ex.gz')
    ex_lr = lr2[1].get_energy() * Hartree
    equal(e_singlet_lr, e_singlet, 0.05)

Support for IVOs in lrTDDFT is done along the work of Berman and Kaldor [BK79].

If the number of bands in the calculation exceeds the number of bands delivered by the datasets, GPAW initializes the missing bands randomly. Calculations utilizing Hartree-Fock exchange can only use the RMM-DIIS eigensolver. Therefore the states might not converge to the energetically lowest states. To circumvent this problem on can made a calculation using a semi-local functional like PBE and uses this wave-functions as a basis for the following calculation utilizing Hartree-Fock exchange as shown in the following code snippet which uses PBE0 in conjuncture with the IVOs:

"""Test calculation for unoccupied states using IVOs."""
from __future__ import print_function

from ase.build import molecule
from gpaw.cluster import Cluster
from gpaw import GPAW, KohnShamConvergenceError, FermiDirac
from gpaw.eigensolvers import CG, RMMDIIS
from gpaw.mixer import MixerDif

calc_parms = [
    {'xc': 'PBE0:unocc=True',
     'eigensolver': RMMDIIS(niter=5),
     'convergence': {
         'energy': 0.005,
         'bands': -2,
         'eigenstates': 1e-4,
         'density': 1e-3}},
    {'xc': 'PBE0:excitation=singlet',
     'convergence': {
         'energy': 0.005,
         'bands': 'occupied',
         'eigenstates': 1e-4,
         'density': 1e-3}}]

def calc_me(atoms, nbands):
    """Do the calculation."""
    molecule_name = atoms.get_chemical_formula()
    atoms.set_initial_magnetic_moments([-1.0, 1.0])
    fname = '.'.join([molecule_name, 'PBE-SIN'])
    calc = GPAW(h=0.25,
                txt=fname + '.log',
                occupations=FermiDirac(0.0, fixmagmom=True),
                    'energy': 0.005,
                    'bands': nbands,
                    'eigenstates': 1e-4,
                    'density': 1e-3,
    except KohnShamConvergenceError:
    if calc.scf.converged:
        for calcp in calc_parms:
            except KohnShamConvergenceError:
        if calc.scf.converged:
            calc.write(fname + '.gpw', mode='all')

loa = Cluster(molecule('NaCl'))
loa.minimal_box(border=6.0, h=0.25, multiple=16)
loa.translate([0.001, 0.002, 0.003])
nbands = 25
calc_me(loa, nbands)
[AB98]C. Adamo and V. Barone. Toward Chemical Accuracy in the Computation of NMR Shieldings: The PBE0 Model.. Chem. Phys. Lett. 298.1 (11. Dec. 1998), S. 113–119.
[Ba94]V. Barone. Inclusion of Hartree–Fock exchange in density functional methods. Hyperfine structure of second row atoms and hydrides. Jour. Chem. Phys. 101.8 (1994), S. 6834–6838.
[BK79]M. Berman and U. Kaldor. Fast calculation of excited-state potentials for rare-gas diatomic molecules: Ne2 and Ar2. Chem. Phys. 43.3 (1979), S. 375–383.
[HA71]S. Huzinaga and C. Arnau. Virtual Orbitals in Hartree–Fock Theory. II. Jour. Chem. Phys. 54.5 (1. Ma. 1971), S. 1948–1951.