[gmx-users] Force constant

André Farias de Moura moura at ufscar.br
Wed Aug 12 17:23:46 CEST 2015

sure, it can be done (not published yet, but we did it for a small molecule
recently - CH2I2):

(1) chose an appropriate level of theory and basis set, perform geometry
optimization and frequency calculation (mind that each combination of level
of theory and basis set have its own scaling factor to correct systematic
errors in frequencies arising from the harmonic oscillator approximation)

(2) you should guess an initial value for the missing force constants, then
you should perform a single molecule, vacuum simulation of your target
molecule (you need to remove PBC, angular and linear momenta an you'll
probably need double precision for that, along with a short time step,
maybe below 1 fs). How long the simulation should be depends on the lowest
frequencies you might be interested in.

(3) calculate the velocity autocorrelation function and take its Fourier
transform, then compare it with the quantum chemical result.

(4) adjust the force constant and repeat steps 2-3 until convergence is
achieved (MM IR spectrum matches QM IR spectrum satisfactorily)

On Wed, Aug 12, 2015 at 11:11 AM, Mehrnoosh Hazrati <
mehrnoosh.hazrati at modares.ac.ir> wrote:

> Dear all,
> I have to write the .itp file for my molecule, and I have problem with
> finding the correct value for force constants.
> Is it possible to calculate bond/angle force constant from ab initio
> calculation?
> Thank you in advance
> Mehrnoosh
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Prof. Dr. André Farias de Moura
Department of Chemistry
Federal University of São Carlos
São Carlos - Brazil
phone: +55-16-3351-8090

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