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I want to calculate the optimal geometry of a molecule adsorbed on a surface. The internal degrees of freedom of the molecule (bond lengths, internal angles etc) are quite easily calculated, but the angle between the molecule and the surface is not properly defined (it is floppy, i.e. the energy difference between two different angles is not large). In Cartesian coordinates, this means that all the positions of all the atoms are changed all the time at each iteration step and this takes a very long time to converge, if it does converge at all. I have heard that there exists a tool to optimize in internal coordinates, just as this is done in SIESTA, but I cannot locate it anywhere. Can anyone help me to confirm if this tool exists, where I can find it and how I can use it? Thanx.

this tool is not included in the officially distributed version of vasp.

But is there such tool available?

Would it be possible to use a tool which can vary the internal degrees of freedom?

I have to check with the people who developed it, it is a tool which is not officially included in the package and not supported therefore.