Calculating the matrix elements given in Eq. (??) consists of two 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 run of evib requires the following control flags:
in the $dft section, usage of large grids is therefore recommended and the multiple
grids m3 to m5 should be avoided.
The elements of the matrix in Eq. (??) are stored in binary format in dfdxi.dat (dfdxi_a.dat and dfdxi_b.dat for UHF), using formatted Fortran output with a record length of 8 bytes for each matrix element. The matrix elements H_{μν,χ}^{e} are stored in the atomic orbital (AO) basis in blocks ordered as (i) (N_{ao} + 1) * N_{ao}∕2 matrix elements of the upper part of the triangular matrix for the μν indices at a certain χ = (k,u), (ii) Cartesian component u = x,y,z, (iii) atom number k.
Optionally,
can be used to generate text output of the matrix elements.
For bigger systems this option can, however, generate very large output files.