# 8.15 Forces and Vibrational Frequencies with ECPs

(May 16, 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 TeO${}_{2}$ 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

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$molecule read$end

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


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