8 Basis Sets and Effective Core Potentials

8.15 Forces and Vibrational Frequencies with ECPs

(June 30, 2021)

It is important to be able to optimize geometries using pseudopotentials and for this purpose Q-Chem contains analytical first derivatives of the nuclear potential energy term for ECPs.

The ECP package is also integrated with the vibrational analysis package and it is therefore possible to compute the vibrational frequencies (and hence the infrared and Raman spectra) of systems in which some of the atoms may bear ECPs. Starting with Q-Chem version 5.0, fully analytical second derivatives of ECPs are available.

Example 8.12  Structure and vibrational frequencies of TeO2 using Hartree-Fock theory and the Stuttgart relativistic large-core ECPs. Note that the vibrational frequency job reads both the optimized structure and the molecular orbitals from the geometry optimization job that precedes it.

$molecule
   0  1
   Te
   O1  Te  r
   O2  Te  r  O1  a

   r = 1.8
   a = 108
$end

$rem
   JOBTYPE    opt
   METHOD     hf
   ECP        srlc
$end

@@@

$molecule
   read
$end

$rem
   JOBTYPE     freq
   METHOD      hf
   ECP         srlc
   SCF_GUESS   read
$end

View output