I would like to do a geometry optimization of a 3-D periodic system using Forcite, keeping my c-parameter fixed and adjusting the a and b parameters. Also, I want to preserve my basic surface structure, so I don't want to optimize the atomic positions (at least, the Cartesian coordinates). So, I figured that I could just select all of the atoms and use "fix fractional position" under Constraints: Atom, and the atomic positions would be "scaled" along with the a and b parameters. Then under Constraints: Lattice, I fix c (as well as the three angles).

The geometry optimization stops with the error: ""Fixed atoms are only permitted if the cell parameters are fixed too. Please fix the cell parameters or remove all atomic constraints". What would be the point of fixing atom positions if the cell parameters are fixed, too? What would actually optimize during the geometry optimization if atom positions and cell parameters are fixed?

From the Help Topic "Support for constraints in Materials Studio,": "Forcite: Supports atom positions fixed in Cartesian or fractional space and fixed lattice parameters, but ignores partial constraints on the x, y, or z components of Cartesian atom positions and geometric constraints. Fractional constraints are not supported for Dynamics calculations."

So, I'm doubly confused by the error, since it doesn't look like I've surpassed Forcite's abilities regarding constraints under geometry optimization.

Also, if I do fix the atom positions and cell parameters according to the suggestion of the error message, I get the following error: "The selected task cannot be performed because all variables are constrained." This (of course) makes complete sense to me, but still leaves me baffled at the first error message.

In what case could one even use "fix fractional positions" in a useful way, if you can't use them in the way I've attempted?

Any ideas?

Thanks,

Todd