This is because MMFF94 (the default force field) cannot handle this molecule type. Open Extensions->Molecular Mechanics->Setup force field and try one of the others -- UFF should work.
I'll leave this open to implement some logic for ff selection. From Geoff on the mailing list:
My suggestion is that Avogadro adopt a "best method" option as the default. That is, if the force field can't be set up for MMFF94, switch to UFF. If someone picks a specific force field, use that and present the error.
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This is because MMFF94 (the default force field) cannot handle this molecule type. Open Extensions->Molecular Mechanics->Setup force field and try one of the others -- UFF should work.
I'll leave this open to implement some logic for ff selection. From Geoff on the mailing list:
My suggestion is that Avogadro adopt a "best method" option as the default. That is, if the force field can't be set up for MMFF94, switch to UFF. If someone picks a specific force field, use that and present the error.
http://github.com/cryos/avogadro/commit/50acbbeba91236a1387a10011438a32ce73ce69e
Force field extension will now switch to UFF if it cannot set up the forcefield.