# Very Small Parameters

Hi everybody

I’m going to simulate a hydrocarbon system using GROMOS force field. In my work, Real was chosen for system unit. In original papers of that paper, bond parameters (K0) are in 10^6 kJ/mol.nm4 and number sounds sensible.
However, when I convert those to corresponding values in Real unit, I’ll obtain very small numbers as small as 10^-10! I can’t find such small values in LAMMPS manual.

Could anybody tell me, am I right?
If so, how to include them in the LAMMPS data file to avoid the risk of round-off error in calculation?

Any hint is welcome.

I would double check the correctness of your unit conversion and/or your understanding of the GROMOS units.

Also, please check this out regarding the round-off errors:
https://en.wikipedia.org/wiki/Floating-point_arithmetic

Hi everybody

I'm going to simulate a hydrocarbon system using GROMOS force field. In my
work, *Real* was chosen for system unit. In original papers of that paper,
bond parameters (K0) are in 10^6 kJ/mol.nm4 and number sounds sensible.
However, when I convert those to corresponding values in *Real* unit, I'll
obtain very small numbers as small as 10^-10! I can't find such small
values in LAMMPS manual.

Could anybody tell me, am I right?

​this sounds wrong. your conversion doesn't make sense. you should get
numbers in the 10^2 kcal/mol range.

axel.​

Thanks Axel

I’ve attached the original paper of GROMOS. Please take a look at table 2, where in the header the bond-stretching constant is 10^6 kJ/mol.nm^4.

So, I convert that unit to Real as follows:

(10^-6)*(1 kcal/4.184kJ)(1 nm/ 10 Ang)^4 ==> (10^-6)(1/4.184)(1/10)^4

I think my calculation is right. But, like you, it sounds wrong!
But, What’s the reason?

Thanks Axel

I've attached the original paper of GROMOS. Please take a look at table 2,
where in the header the bond-stretching constant is 10^6 kJ/mol.nm^4.

So, I convert that unit to Real as follows:

(10^-6)*(1 kcal/4.184kJ)(1 nm/ 10 Ang)^4 ==> (10^-6)(1/4.184)(1/10)^4

I think my calculation is right. But, like you, it sounds wrong!
But, What's the reason?

​you copied the paper wrong. it is 10^6 *NOT* 10^-6

axel.​

OK!

Seemingly, the paper notation has been wrong!

As far as I know, when a number is so small, the authors make use of such notation to reduce decimal
​ ​
numbers.
​ Am I right Axel?

OK!

Seemingly, the paper notation has been wrong!

​the notation in the paper makes sense and seems correct to me.

axel.​