Requests a density-functional based tight-binding semi-empirical calculation, a method which is parametrized via the results of DFT calculations. DFTB uses the tabulated matrix elements as in the original implementation of Elstner and coworkers [Porezag95, Elstner98]; DFTBA is a version which uses analytic expressions for the matrix elements rather than tabulated ones [Zheng07]. See [Frauenheim00, Frauenheim02, Zheng05, Sattelmeyer06, Otte07] for review articles and calibration studies.
There are no internally stored parameters for these methods, so one of the following options must be specified.
Read values for parameters from the input stream.
Read parameters from the checkpoint file.
Energies, gradients and frequencies.
DFTBA is parametrized for all pairs of H, C, N, and O. It is also parametrized for the metals Sc, Ti, Fe, Co, and Ni but only with H, C, N, and O. That is, Fe5CO and Sc5CO are supported, but Fe4ScCO is not. The DFTB parameter files are copyright by Professor Elstner and must be obtained from him.
The following input file format runs a DFTBA calculation using the parameter set provided with Gaussian 09:
# DFTBA OPT FREQ
Ala3 DFTB frequencies
Molecule specification continues …
For DFTB, the same format of parameter files is used as in other programs: one file for each pair of elements, with the order of the two elements being significant. Accordingly, a calculation on H2CO would use a parameter input section something like this:
The DFTB energy appears in the output as follows:
SCF Done: E(RDFT-SCTBA) = -33.9465130617 A.U. after 11 cycles
DFTB Parameter (.skf) Files and Gaussian 09
The handling of DFTB input files has been modified for compatibility with the files provided by Elstner: HTML data at the end of the file is ignored. Also, multipliers—e.g. 10*1.0—are now accepted.
MODIFYING SLATER-KOSTER FILES (.skf) FROM dftb.org FOR USE WITH GAUSSIAN 09
- The first line of each file must be edited to identify the two elements involved. For example, in the file H-S.skf the first line should contain the atomic numbers, so it goes from dftb.org format:
to Gaussian 09 format:
0.02, 500 1 16
- Some of the files involving hydrogens are links rather than separate files, because they are same as the corresponding file with the order of the elements reversed. For example, H-C.skf is a link to C-H.skf. However, Gaussian 09 expects both files to be provided, differing only in the order of the atomic numbers on the first line.
COMPUTING SECOND DERIVATIVES WHEN USING PARAMETER FILES
Analytic second derivatives are available in G09 when the analytic DFTB parameters (provided with the program) are used, but are not available when the tabulated parameters from dftb.org are used. This is because the linear interpolation which is done using the tabulated parameters give discontinuous first derivatives at first points, so the second derivatives do not always exist. If you want to ignore this potential problem and compute second derivatives using the tabulated parameters, then Freq=Numer must be specified in the job's route section.
Last update: 13 May 2013