# [Gmsh] numeric scale questions

Steve Talent talent at analograils.com
Mon Sep 6 02:11:31 CEST 2010

```Hi,

I have a some questions regarding numeric scales in geometry and mesh
construction.

I'm using the gmsh scripting language to generate geometry. I generate
the '.geo' scripts from a program. The geometry describes a "fringe
capacitor" structure used in integrated circuits. I'm using the
electrostatic field solver in ElmerFEM to calculate electric fields and
capacitance.

I'm using MKS units to define the geometry. The overall volume is on the
order to 10 microns in x, y, and z directions. Geometric features may be
as small as 30 nanometers.

So the numeric values I'm using to describe geometry and minimum
characteristic length are at a scale of ~ 1e-8.

The questions:

Are the algorithms using relative or absolute precision?

Is it safe to define geometry with numeric values in the range 1e-8 to
1e-5?

Should I scale my model to a different range of values?

Are there parameters I should adjust to accommodate working with the
range of a dimensional values in my model instead of scaling?

Following is additional information which may or may not be relevant:

By the way, I made a mistake one time where I had...

Field.FieldsList={3,4};

... which resulted in run-away recursion. User error? Yes. Bug? Maybe --
should be easy to detect.

I'm experimenting with fields to control meshing.

Field=Attractor;
Field.NodesList={attractorPoints[]};
Field=Threshold;
Field.IField=1;
Field.LcMin=2.75e-08;
Field.LcMax=2.85e-07*2;
Field.DistMin=0;
Field.DistMax=4.48286e-06;
Field=Box;
Field.VIn=2.75e-08*2;
Field.VOut=2.85e-07;
Field.XMin=-3.355e-06;
Field.XMax=3.355e-06;
Field.YMin=-2.84e-06;
Field.YMax=2.84e-06;
Field.ZMin=5.9e-07;
Field.ZMax=2.35e-06;
Field=Min;
Field.FieldsList={2,3};
Background Field = 4;
Mesh.Algorithm = 5;
Mesh.Algorithm3D = 1;
Mesh.CharacteristicLengthExtendFromBoundary = 0;

Thanks!

Steve

```