15.3 General usage of evib

Calculating the matrix elements given in Eq. (15.4) consists of two steps. First a force constant calculation using aoforce is performed, where the following control flags have to be added:

$nosalc

,

$sijuai_out


This will save the derivative of the density matrix.

The subsequent run of evib requires the following control flags:

no weight derivatives


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. (15.4) 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 Hμν,χe are stored in the atomic orbital (AO) basis in blocks ordered as (i) (Nao + 1) *Nao2 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,

$dfdxi textout


can be used to generate text output of the matrix elements.

For bigger systems this option can, however, generate very large output files.