Internal (Z-Matrix) to Cartesian coordinates conversion

Is there any way to convert from Internal to Cartesian coordinates using simple formulas?


I have studied and implemented: [INTERNAL TO CARTESIAN](http://dx.doi.org/10.1002/9783906390390.app2) by Jack D. Kunitz.


But somehow the x, y and z values are not correct. I am not implementing the transformation back to the initial frame as it says, because I do not understand that step. Is that the reason for not being able to get correct answers or is this not a good tutorial or solution to follow.




Sure, it's just trigonometry. There are a variety of open source implementations.


For example: 


* [InternalToCartesian from Open Babel](https://github.com/openbabel/openbabel/blob/master/src/obutil.cpp#L439)
* [ChemKit](https://github.com/kylelutz/chemkit/blob/master/src/chemkit/internalcoordinates.cpp#L209)
* [QCL Python](https://github.com/ben-albrecht/qcl/blob/master/qcl/ccdata_xyz.py#L208)


There are dozens in a search on [GitHub](http://github.com/) if you don't like C++ or Python.


There's also a web app from [Shodor](http://www.shodor.org/chemviz/zmatrices/babel.html) for z-matrix conversion.


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