harmonic and cubic force constants

[payam]

Dear ATK users,
Does anyone know how to calculate harmonic and anharmonic (cubic) force constants using ATK-classical module?

Best regards,
Payam

[Jess Wellendorff]

Perhaps it is the Dynamical Matrix you are looking for?
http://docs.quantumwise.com/manuals/Types/DynamicalMatrix/DynamicalMatrix.html#notes

[payam]

It seems that it calculates second order force constants only.

[Jess Wellendorff]

I don't think we ATK  does 4-th order derivatives of the energy wrt. nuclei positions.

[Anders Blom]

You can extract the phonon DOS with anharmonic effects from an MD simulation, but not the force constants.

[Petr Khomyakov]

In principle,  anharmonic interatomic force constants can be extracted from molecular dynamics (MD) simulations, e.g., take a look at the following papers http://iopscience.iop.org.globalproxy.cvt.dk/article/10.1088/0953-8984/26/22/225402/meta and http://journals.aps.org/prb/pdf/10.1103/PhysRevB.77.144112.

This extraction is not automatic in the VNL-ATK. You should first do DFT or classical MD calculations in ATK for the system of interest, and then follow the extraction procedure described in the references enclosed. For classical MD, you have to make sure that the classical potential you have adopted for MD accounts for anharmonic effects.