Interface to the DFTB+ program implementing self consistenct charges density functional tight binding (SCC-DFTB) methods.[1] The program requires also parameters available separately at www.dftb.org. Open source software.
Feb 2, 2016: two important changes were made in the input. Firstly, the built-in DFTB-D dispersion is now switched off by default. Warning is printed when the keyword is not set for a transition period. Secondly, the dftb_hubbard_derivs behaves differently and the default setup was changed to the values used in DFTB3.
Empirical dispersion coorrection was developed for SCC-DFTB with the MIO parameters set.[2] It is available in DFTB+ and it can be activated by the dftb_dispersion keyword.
Since DFTB+ version 17.1, the D3 correction is available in the program. It is activated using the keyword dftb_d3. The damping is controlled by several keywords borrowed from the dispersion3 interface.
The acronym DFTB3 is used for a third-order SCC-DFTB with the 3OB parameter set and gamma function damping exponent of 4.0.[3] The folowing input reproduces the DFTB3 setup in cuby:
method: scc-dftb3
dftb_slko_set: 3ob
dftb_xh_damping: yes
dftb_xh_damping_exp: 4.0
The D3 dispersion was parameterized for DFTB3 by Grimme[4], in Cuby it can be applied using the following setup:
modifiers: dispersion3
modifier_dispersion3:
d3_damping: bj
d3_a1: 0.746
d3_a2: 4.191
d3_s8: 3.209
In the latest release of DFTB+, the built-in D3 dispersion correction can be used; the results should be identical. The input is slightly different, the dispersion setup is passed directly to the dftb interface. The example abowe would change into:
method: scc-dftb3
dftb_slko_set: 3ob
dftb_xh_damping: yes
dftb_xh_damping_exp: 4.0
dftb_d3: yes
d3_damping: bj
d3_a1: 0.746
d3_a2: 4.191
d3_s8: 3.209
The latest version of the standalone D3H4 corrections[5] was recently reparameterized for DFTB3, making the DFTB3-D3H4 method.[6] Here, a different parameterization of the D3 dispersion is used which contains also a repulsive correction between hydrogens. The DFTB3-D3H4 calculation can be called using the following input:
method: scc-dftb3
dftb_slko_set: 3ob-3-1
modifiers: dispersion3, h_bonds4
# Dispersion3 setup
modifier_dispersion3:
d3_damping: :zero
d3_sr6: 1.25
d3_alpha6: 29.61
d3_s8: 0.49
# Hydrogen-hydrogen repulsion:
d3_hh_fix: yes
d3_hh_para:
:k: 0.30
:e: 14.31
:r0: 2.35
# H4 correction setup, including the additional
# scaling for charged systems
modifier_h_bonds4:
h_bonds4_parameters:
oh_o: 1.28
oh_n: 3.84
nh_o: 0.88
nh_n: 2.83
multiplier_wh_o: 1.00
multiplier_coo: 1.75
multiplier_nh4: 4.01
h_bonds4_extra_scaling:
"@N&:hip": 3.44
"@N&:gua": 2.68
The latest version of hydrogen bond correction is embedded in the DFTB calculation, and it is thus implemented direcly in the DFTB+ program. If it is compiled with the support of D3 dispersion (which is not included in the provided binaries), the complete DFTB3-D3H5 calculation can be preformed in DFTB+ using a following setup:
interface: dftb
method: scc-dftb3
dftb_slko_set: 3ob-3-1
dftb_h5: yes
dftb_d3: yes
dftb_d3_hhrep: yes
d3_damping: :zero
d3_sr6: 1.25
d3_alpha6: 29.61
d3_s8: 0.49
The interface now supports the COSMO solvent with default setup (optimized COSMO radii) and solvent selected by name using keyword solvent_name.