[gmx-users] Force constant

[Mark Abraham]

Hi,

To which I would also suggest adding checks of any available experimental
quantities (e.g. density, heat of vaporization) and considering carefully
following the techniques used in parametrizing the parent force field.

Mark

On Wed, Aug 12, 2015 at 5:24 PM André Farias de Moura <moura at ufscar.br>
wrote:

> 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
> > --
> > Gromacs Users mailing list
> >
> > * Please search the archive at
> > http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before
> > posting!
> >
> > * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
> >
> > * For (un)subscribe requests visit
> > https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or
> > send a mail to gmx-users-request at gromacs.org.
> >
>
>
>
> --
> _____________
>
> Prof. Dr. André Farias de Moura
> Department of Chemistry
> Federal University of São Carlos
> São Carlos - Brazil
> phone: +55-16-3351-8090
> --
> Gromacs Users mailing list
>
> * Please search the archive at
> http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before
> posting!
>
> * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
>
> * For (un)subscribe requests visit
> https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or
> send a mail to gmx-users-request at gromacs.org.