CN: Analytic Frequencies

Sample directory: adf/CN_SecDeriv/

The ADF2002.01 version featured analytic second derivatives (SD) for the first time. This initial implementation had severe limitations, both in terms of speed, as in terms of user-friendliness of the output and the number of available options. As of ADF2006.01, the implementation has been significantly improved. More specifically:

• what was formerly a separate SD program is now part of the main ADF executable;
• most current GGA's have been implemented, with the exeption of PW91;
• the speed of the code, serial as well as parallel, has been improved;
• full support for symmetry has been implemented, including linear molecules.

Calculation of analytical second derivatives is requested by specifying

AnalyticalFreq
End

A high accuracy is specified for the numerical integration to be sure of reliable results.i 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

AnalyticalFreq
End

End input
eor

After SCF is completed, the energy second derivatives matrix is calculated and analysed, which yields in this case one frequency.