# [gmx-users] Force Constants and Unit Systems

Justin A. Lemkul jalemkul at vt.edu
Thu May 17 19:10:37 CEST 2012

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On 5/17/12 12:58 PM, Lara Bunte wrote:
> Hi
>
>
>> One cannot convert between these units, since kcal/mol is an energy term, and
>> kJ/(mol nm^2) is a force constant.
>
>
> This confuses me, because in the paper where that constants are from is written, I quote:
>
> "Force constant k in kcal / mol calculated by DFT"
>
>
> In my parametrization I have this values. Could you please explain this?
>

If this is a force constant for an angle, then it needs to simply be converted
to kJ/mol.  See Table 5.5 of the manual for all force constant requirements.
Your original question is not solvable; one cannot convert kcal/mol to kJ/(mol
nm^2) as you cannot create a distance term out of nothing.

-Justin

> Greetings and really thanks for your help
>
> Lara
>
>
>
>
>
> ----- Ursprüngliche Message -----
> Von: Justin A. Lemkul<jalemkul at vt.edu>
> An: Lara Bunte<lara.bunte at yahoo.de>; Discussion list for GROMACS users<gmx-users at gromacs.org>
> CC:
> Gesendet: 18:52 Donnerstag, 17.Mai 2012
> Betreff: Re: [gmx-users] Force Constants and Unit Systems
>
>
>
> On 5/17/12 12:25 PM, Lara Bunte wrote:
>> Hello
>>
>> I never had contact with such units. Could you please help me and explain me
>> how to transform following units:
>>
>> 1.) kcal / mol -->   kJ / (mol * nm^2)
>>
>
> One cannot convert between these units, since kcal/mol is an energy term, and
> kJ/(mol nm^2) is a force constant.
>
>> 2.) kcal / (mol * rad^2) -->   kJ / (mol * rad^2)
>>
>
> The transformation between kcal and kJ requires a simple multiplication.
>
>> I have additionally some question to that:  Both should be units of a force
>> constant. From Hooke's law F = -kx follows for me, that the unit of a force
>> constant is Newton N over distance m, in S.I. units  kg/sec^2 (mass over time
>> square).
>>
>> What is the correspondence between above from Hooke's law and that units of
>> force constants in molecular dynamics simulations?
>
> Force is also expressed as kJ/(mol nm) - see Chapter 2 of the Gromacs manual.
> Thus, since x is in units of nm (distance), then k has units of kJ/(mol nm^2).
>
> -Justin
>

--
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Justin A. Lemkul, Ph.D.
Research Scientist
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080
http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin

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