# Using “soft” constraints / bias in geometry optimization?

I study molecules weakly bound to nanoclusters and surfaces. During geometry optimization, I often run into problems like for example a small metal cluster behave nicely and has a given, stable geometry for 90% of the calculations, then in the remaining 10% of cases, it reorganizes to some other geometry. On one hand, these problems are expected as there are several different stability points around, often with relatively low transition barrier, on the other hand, it is rather annoying when changing a functional or basis set, or something minor chemical change induces such a transformation and makes impossible any comparison between geometries. One can use a geometry constrain to fix the geometry every time, but I feel kind of uneasy about it.

I was wondering if there is an easy way to introduce an extra force, similarly like biases can be introduced in MD simulations to do meta-dynamics, so it smoothly lowers the energy of a given geometry and ideally makes it stable. Technicalities: I am using molecular codes (Gaussian, ORCA) for such calculations at the DFT level.

• What will be the difference between fixing the geometry and a adding a (fictional) extra force? For me, both are the same. – Camps Mar 5 at 11:51
• I guess I just do not like to fix cartesian coordinates when e.g. I expect changes in bond length (which can happen when you change functional or basis set). – Greg Mar 5 at 12:01
• I think you will find it nearly impossible to do this in a useful way. You may have some luck with applying a constraint, optimizing, then releasing the constraint. This lets you start at a more optimum structure, closer to the local minimum you are trying t o hit – Tristan Maxson Mar 5 at 15:20
• In Orca, if you are starting the optimization with a guess hessian, you can set the force constant of a particular coordinate (bond, angle, or dihedral or Cartesian). For example %opt Hess_Internal {A 2 1 0 D 2.0} reset 5 end will set the diagonal hessian value for the angle between 2nd, 1st and 0th atom to 2 Eh/Bohr^2 and reset it after 5 optimization steps. You can increase the value to exert more force. – Shoubhik R Maiti Mar 5 at 17:51
• – Martin - マーチン Mar 7 at 14:49