# Sensible default for covalent radius
**URL:** https://discourse.vtk.org/t/sensible-default-for-covalent-radius/2648
**Category:** Support
**Created:** [February 18, 2020, 12:31pm UTC](https://discourse.vtk.org/t/sensible-default-for-covalent-radius/2648 "2020-02-18T12:31:49Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Jellby](https://discourse.vtk.org/u/Jellby)
#### Post date: [February 18, 2020, 12:31pm UTC](https://discourse.vtk.org/t/sensible-default-for-covalent-radius/2648/1 "2020-02-18T12:31:49Z")
I’m using vtkMolecule and vtkSimpleBondPerceiver to display reasonable structures for molecules. But there is a problem when a particular atom does not have a covalent radius specified in the default database: the radius becomes something like 1e38 and the atom appears bonded to every hydrogen in the system. See the following image, where the central atom is Dy (Z=66):
So, first thing, I think the database should be updated (see e.g. [https://periodictable.com/Properties/A/CovalentRadius.v.log.html](https://periodictable.com/Properties/A/CovalentRadius.v.log.html)), and even when there is no data, at least some reasonable value should be used as default (e.g. ~200 pm).
Secondly, how can I specify custom radii for a molecule? I could subclass vtkPeriodicTable, but how can I force vtkSimpleBondPerceiver to use the subclass?
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### Author:  [@nicolas.vuaille](https://discourse.vtk.org/u/nicolas.vuaille)
#### Post date: [February 20, 2020, 4:00pm UTC](https://discourse.vtk.org/t/sensible-default-for-covalent-radius/2648/2 "2020-02-20T16:00:12Z")
> [@Jellby](#):
>
> how can I specify custom radii for a molecule?
Not sure which radii you are talking about. Atoms Covalent Radii ?
I fear it is not possible without modifications in VTK itself.
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### Author:  [@Jellby](https://discourse.vtk.org/u/Jellby)
#### Post date: [February 20, 2020, 4:10pm UTC](https://discourse.vtk.org/t/sensible-default-for-covalent-radius/2648/3 "2020-02-20T16:10:25Z")
Yes, covalent radii according to the vtkSimpleBondPerceiver description. I’ve created a VTK issue: [https://gitlab.kitware.com/vtk/vtk/issues/17791](https://gitlab.kitware.com/vtk/vtk/issues/17791), but you already know, I guess 😉
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### Author:  [@nicolas.vuaille](https://discourse.vtk.org/u/nicolas.vuaille)
#### Post date: [February 21, 2020, 10:58am UTC](https://discourse.vtk.org/t/sensible-default-for-covalent-radius/2648/4 "2020-02-21T10:58:44Z")
Yup.
This merge request ([https://gitlab.kitware.com/vtk/vtk/merge\_requests/6494](https://gitlab.kitware.com/vtk/vtk/merge_requests/6494)) updates the database, and should fix your extra-bonds problem (as there is no more undefined values).
Specifying a custom radii should not be very difficult, it can be a good first contrib to VTK for anyone 😉
(I don’t really have time to do it for now)