16.3 General usage of evib

Calculating the matrix elements given in Eq. (16.4) 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, which are necssary for the subsequent evib run.

The matrix elements of \(H^{e}_{\mu\nu,\chi}\) (Eq. (16.4)) are stored by default in binary format in the file 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 \(H^{e}_{\mu\nu,\chi}\) with \(\chi=(k,u)\) are in the atomic orbital (AO) and ordered as follows: (i) Cartesian component \(u=x,y,z\), (ii) atom number \(k\), and (iii) \((N_{\text{ao}}+1)*N_{\text{ao}}/2\) matrix elements of the upper part of the triangular matrix for the \(\mu\nu\) indices.

Optionally,

$dfdxi textout

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

Additonally the first derivatives of the orbital energies (Kohn-Sham eigenvalues) with respect to the atomic displacements \(\textrm{d}\epsilon_{i}/\textrm{d}\chi\) are calculated and stored in the text file dEidR.dat (dEidR_a.dat and dEidR_b.dat for UHF).