QuantumATK Forum

QuantumATK => General Questions and Answers => Topic started by: payam on September 19, 2016, 21:55

Title: harmonic and cubic force constants
Post by: payam on September 19, 2016, 21:55

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

Best regards,
Payam

Title: Re: harmonic and cubic force constants
Post by: Jess Wellendorff on September 20, 2016, 13:49

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

Title: Re: harmonic and cubic force constants
Post by: payam on September 20, 2016, 17:01

It seems that it calculates second order force constants only.

Title: Re: harmonic and cubic force constants
Post by: Jess Wellendorff on September 21, 2016, 14:14

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

Title: Re: harmonic and cubic force constants
Post by: Anders Blom on September 27, 2016, 00:11

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

Title: Re: harmonic and cubic force constants
Post by: Petr Khomyakov on September 27, 2016, 10:01

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.