Calculating the matrix elements given in Eq. (14.4) consists of 2 steps. First a force constant calculation using aoforce is performed, where the following control flags have to be added
This will save the derivative of the density matrix. The subsequent evib requires the following control flags
in $dft section, usage of large grids is therefore recommend and mgrid should
be avoided.
The elements of the upper part of triangular matrix Eq. (14.4) are stored binary in dfdxi.dat (dfdxi_a.dat and dfdxi_b.dat for UHF) using formatted fortran output with record length of 8 bytes for each matrix element. The matrix elements are stored in the AO-basis in blocks ordered as "1. (N_{ao} + 1) * N_{ao}∕2 matrix elements, 2. cartesian component {x,y,z}, 3. atom number". Optional
can be used to generate text output of the matrix elements. For bigger systems
this can however generate very large output files.