Sample directory: adf/e_CN_SecDeriv/
he ADF2002.01 version features analytic second derivatives (SD) for the first time. This initial implementation still has severe limitations though, both in terms of speed, as in terms of user-friendliness of the output and the number of available options. For most ADF users, it will still be recommendable to use the finite difference implementation which can be invoked through the FREQUENCIES subkey in the GEOMETRY block (see ADF User's Guide for details).
As in other property programs, first a preparatory ADF calculation needs to be done. This calculation will usually take much less time than the SD calculation. There are a number of restrictions to the ADF run. The line
symmetry NOSYM
is mandatory. The X-alpha potential is specified. Although the programs will accept other potentials, such as GGAs, the user should realize that only the X-alpha functional is currently implemented in the SD program. Although the practical results resulting from a mixed GGA/X-alpha calculation may be very good, please realize that the resulting frequencies will neither be identical to pure GGA, nor the X-alpha frequencies.
Further note that in this example, and many others, the new convenient BASIS keyword is used, which circumvents the need to specify separate Create runs for the atoms.
A high accuracy is specified for the numerical integration to be sure of reliable results. In general, it seems advisable to use high accuracy for heavy nuclei at the moment, whereas default integration accuracy is usually sufficient for light atoms. Further, high integration accuracy is more needed in the atomic spheres than in the rest of the molecule. A cost-effective solution may therefore be to specify a higher integration accuracy in the spheres only (using the accsph subkey of the INTEGRATION keyword).
$ADFBIN/adf << eor title CN atoms N -1.3 0.0 0.0 C 0.0 0.0 0.0 end Basis Type DZ Core None End charge -1 XC LDA Xonly End integration 6.0 symmetry NOSYM End input eor
After the ADF calculation is finished, the SD program reads the TAPE21 result file (as well as possibly other ADF result files) and starts its lengthy calculation. The print keyword obviously gives timing information. The SD keyword specifies that both the GRADIENT (analytic first derivative of energy with respect to nuclear displacements), the HESSIAN (analytic second derivative), and the DIPOLE derivatives are to be calculated and printed. The latter give access to the IR intensities, whereas the HESSIAN gives access to the IR frequencies.
The SD program can run in parallel.
$ADFBIN/sd << eor print timing integration 6.0 SD CALC HESSIAN DIPOLE GRADIENT END eor
